Why could Data not use contractions?

I'd like to point out a possible alternative explanation, although I do not believe it to be official canon: English is hard.

Data had to learn how to do almost everything. While he almost certainly had basic English programmed into him, I find it more likely that contractions weren't necessarily programmed in, beyond (possible) the most common ones. The rest, he's had to actually learn, and is having difficulty with.

One reallife example is human children and English conjugations (This is an exaggerated/paraphrased example; I couldn't find the article I read this in):

• When first learning how to speak, kids start by repeating their parents: "He wants the ball" / "I want the ball".
• Later on, it sometimes seems like kids suddenly forget their grammar: "He want the ball" / "I wants ball" / "Dad want ball"
• Later still, they begin to figure out the patterns: "I want the ball" / "Dad wants the ball". (Well, they should. I knew people in highschool who still couldn't figure it out...)

The "forgetting" is because instead of just repeating the phrase by rote, they're trying to figure out why it's "want" instead of "wants". It's a step towards ingraining the rules of grammar so that it's automatic.

I believe that throughout TNG, Data is somewhere between points 1 and 2. Soong had programmed in several contractions for Data to make use of, automatically replacing words with their contractions if he was going to say them aloud. Data had, by the time of TNG, been unconsciously replacing some of those protocols with the more formal form, in an attempt to learn the patterns involved so that they could be applied more generally.

Point 1 is supported in that Data's contractions are often either some common phrase ("I'm fine."), repeating something someone else just said (where he doesn't have to figure out anything), or manually programmed into himself ("Honey, I'm home!").

Point 2 is supported by Data's use of the formal form becoming more common as the series progressed. After Lal pointed it out to him, it could even have become more ingrained, something he was doing unconsciously to be more unique. (Part of what drove Data was to become more human, and one of the key qualities about humans is that we're all different from each other, while machines are not)

Point 3 is supported in the series finale of TNG, in the timeline where Picard is in the future - Data can fully use contractions in everyday speech.

There's an instance I haven't seen on ANY comprehensive list of Data's "contraction slip-ups", in season 6 ep 21 "Frame of Mind" around 27 seconds into the episode where Data says, "You mean you don't think you should stand trial..."

This contraction really got me thinking, since it's not a sentence Data's coming up with on his own. It's a line in a script he's reading. Any thoughts on how his brain would work? Obviously he can memorize a string of sounds and repeat it, even if those sounds are contractions. But if he can master THAT, why can't he use contractions as parts of common whole phrases?

I am not a native speaker of the English language, and as such, speak with what would be considered a heavy foreign accent. I believe my condition can explain Data's. I can imitate the US accent pretty well, and have had native speakers praise me on occasion. But, I can only do this while doing something relatively trivial, such as engaging in social conversation (small talk). When doing something mentally taxing, such as teaching physics, I have to revert back to my heavy foreign accent, because I just do not have the mental capacity (literally, processing power) to mentally translate each sound in time for normal speed speech, as well as perform whatever intricate computation I am required to on the board and in my mind.

Believe me, regardless of how closely and correctly I try to match a mode of speech not natural to me, and how well I succeed, it will always be a taxing computation.

And, oh yes, while teaching physics, I drop contractions!

Some of the other answers describe the problem, but not the reason.

tl,dr: Data was nerfed to make him more appealing to humans. Humor, emotion and this minor language problem, possibly his love for humans was a trick too.

Data's twin homebro, Lore, is pretty much identical to Data. The thing is that he's a robot which is better than humans in every way. That made him pretty arrogant (he is made to be human, emotions included), and that made him disregard humans.

That and probably his tendency to lie, trick, make fun of people and demonstrate his superiority made the settlers on the colony he was made (by doctor Suung, genius outcast professor) pretty fucking worried. They really kinda just wanted Suung to turn the thing off.

Soong was sad, of course, but Lore contacted the "Cristalline entity" through kind of vague means. Of course that entity was far superior to humans (Source: Lore). Coincidentally it feeds off the lifeforce of humans, entire planets at the time if it can. It was also really fast, like warp fast. Cool thing. Offtopic more here.

Lore contact the entity before he was disassembled. Soong made his second robot with deliberately impaired features like his emotional response, as to not frighten the settlers. See also the uncanny valley, on why that might've helped. Seemingly Soon also fixed the extreme evilness that Lore had, possibly through introducing obedience or admiration of humans. Which would explain Data's goal of having the human experience.

So: Data was made not to use contractions by Soong, possible just as an unimportant gimmick to make humans like him more. Along with other pinches in his capacities elsewhere, like his understanding of sarcasm, inability to laugh, etc.

It was shown that Lore and Data have the same capacities other than in those areas where Data was "pinched". I guess Data is better as he's not adversarial and delusional, although it makes him illogical in many ways. The superiority discussion, of course, could be done later. One also wonders why there's only two prototypes, given Soong still lives.

But hey, that's Star Trek. It works 3 steps of thought more than almost anything else, tells you something about yourself and society without talking about you, and gives you hope for humanity. But a step more the seams start to crack. Why isn't the teleporter all powerfull? Why isn't a superhumancomputerhybrid king of the universe (they actually did that one), etc. Offtopic again. Sorry again. I'm going to stop talking now.

As a programmer, I can semi-humorously say that it just could be an "oops". After all, one guy designed and programmed Data. I'm sure that "English contractions" were way down at the bottom of his list of things to code.

Though many aspects around this is covered by other answers it is also worth mentioning that the Writers Guide specifically mention that Data uses a more formal version of English then the rest of the crew. Directly quoted:

[…] Generally, Data speaks in a more formal brand of English that the rest of our starship crew. He usually avoids contractions like "can't" or "wouldn't." […]

This support the fact that he can use contractions, but usually avoids it.

Of course, as the Writers Guide was written before the series started, the aspect of this fact get more complex as the universe grows. Another detail about Data's nature is described further in the guide by:

What is most appealing about Data is not that he is a machine, but that he is a machine who is eager and enthusiastic about adventure of life. To that end, he is an inquiry into the question, "What does it mean to be a human being?" […]

This is one of the reasons, I believe, we see the language Data have. A central part of being human is language. Further one thing that is special and that require more then pure linguistic skills are for example the use of idioms. Understanding language beyond the pure glossary and grammar is one of his quests.

One early encounter on idiomatic expressions is when Riker asks Data do do a data search.

RIKER: I know... like looking for a needle in a haystack.

DATA : (puzzled) Why should anyone wish to expend their time in such a search?

RIKER: Correction, Data, I should have said proverbial needle in a haystack.

DATA: Ah, a human proverb! As in folklore, or an historical allusion, or tribal memories, or...

Data is in many ways shown to be like a highly educated / having a vast memory bank as well as mastering languages etc. but very often it also comes forth as cold facts and very correct. When it comes to language specifically it is formal by which this correctness is mirrored.

The formal language is believably also a consequence of Data being set to hold and communicate facts. The formality given is something you also expect in for example a lexicon, a formal letter or other text. One would not expect to read: "Adolse was king of Nagaria for twenty years and ruled it with an iron fist. As a party animal it was only fit he kicked the bucket one fatal night whilst painting the town red."

All of this also makes the use of contractions, as in lack there of, part of what one expect.

Just my 2 cents in this forever old conversation. In his offspring episode, Data is surprised that she uses contractions so easily while he then states he still has a hard time comprehending their use. Then, be it a lack of attentive writing, or maybe a slight nod to the previous scene (assuming he's trying now to use them more) he immediately uses several contractions in his conversation with Picard. I think Data's self analysis of the difficulty he has with contractions is a mixture of what others have said, the contractions aren't "formal" per se. But, I'd also add the more logical reasoning is the infinite ambiguity one can use in choosing their words which allows you to avoid contractions entirely and often times, at least for Data (who is near infinitely resourceful) may be a more effective way to communicate. I say all of this with regard that Data is an android, computing responses before they are required. That is to say you need to consider Data's speech patterns have nothing to do with his own comfort in his ability to speak, but rather who he's talking to, what about, how they are reacting... it's a more sci-fi idea behind it but I like to think just talking for Data, is far more than just talking, for him.

As far as programming had progressed at the time of producing TNG -- there may have been issues with string literals '' and preserved string literals "", where command substitutions using ` instead of ' may have had potential conflicts with the universal translator as well as '' being very similar to " -- take the commands sed and awk for example .. 's is the beginning of text substitution for sed and 't the trimming of text for gawk. This is makes sense as Soong knew the value of data and the unintentional exploits that could possibly generate through information corruption .. it is evident as Data would definitely have security subroutines reiterating to prevent these problems, possibly nested inside an endless recursion function.