"need to do" vs "need do"

Question

Consider:

I need to do this.
I need do this.

My English grammar knowledge tells me that "need" doesn't have the same status as the modal verbs "may", "can", "should" and what not. Hence the second usage where two verbs appear consecutively is incorrect.
But yesterday, my native English speaking friends (Americans) told me that "I need compute this." is a perfectly grammatical sentence, and one is simply omitting the "to". How can this be? Is it a colloquial usage but grammatically incorrect, or is it grammatically correct? If it is grammatically correct, is it because "need" is a semi-modal verb?
EDIT: in particular, is it okay to use "need compute" in a scientific paper?
EDIT2: the exact phrase that raised the question was "The advantage of this representation is that we need only compute sums and products"

Peter Shor · Accepted Answer

Your friend is correct. "I need compute ..." is ungrammatical, but "I need only compute ..." is fine, if a little bit old-fashioned and formal.

Modal verbs do not use a "to". That is, you say

I can do this.

The verb "need" is a funny case; it is only modal in the negative. In the positive, we already have an equivalent modal verb; namely, "I must". However, there are two possible meanings for the opposite of "I must do this": "I am forbidden to do this" and "I am not required to do this". These two different meanings are conveyed by the modal verbs "I must not" and "I need not".

Searching with Google books, it appears to have been this way at least since 1600 (although back then, there was a positive construction "I must needs", which has since for the most part fallen out of use). Thus, you get various grammatical constructions.

In the negative, you have:

I need not do this.
  I do not need to do this.

In the positive, you have:

I must do this.
  I need to do this.
  *I must needs do this. (obsolete)

"I need do this" is incorrect.

Over the last few centuries, "I don't need to" has slowly been replacing "I need not", but "I need not do this" is still used reasonably frequently, and is grammatical. However, if "I need do this" was ever grammatical, it was in the long distant past.

Finally, in the past you could say

it is not the case that I need compute this,

since that is a negative use (and this is why your friend might not be wrong). Today, I believe most people would use "need to" here. But if your friend was using "need compute" in the negative, there is a good case to make for it being grammatical.

Patrick T. Randolph · Answer

"Need to" is one of the English modals. "I need to do this" is the more common usage and the one used most commonly in the States. "I need do this" is a bit affected use of the expression. Hope that helps.

Answered by Patrick T. Randolph on March 13, 2021

terdon · Answer

Simply put, your friends are wrong. Unfortunately, being a native speaker does not mean that one can speak a language correctly. Far from it. Indeed, it is often the case tat those who speak a language best are those who have learned it (very well) as a second language. They tend to be more aware of the grammatical rules. I remember a Scottish friend trying to convince me the following sentence is correct English:

Where has Dave went?

That said, your friends may be confused by an archaic idiom. For example, a character in Shakespeare's Hamlet says:

Such love must needs be treason in my breast.

This, however, is both archaic and different from needs compute. You could, if consciously trying to be pretentious say

[...] must needs be computed.

jpillow · Answer

There's something missing from the description of the problem, which is the omission of the word only.  I grant that it is an archaic construction, but I do not concede it is incorrect.

1.  In this setting, we need only consider X.

We could equivalently say

2.  In this setting, we only need to consider X.

I agree that the second is more common, but would submit that the first is not incorrect.

I'm adding a note forwarded by "friend #2".  In fact, it appears that only does indeed affect grammaticality.

According to Swann (Practical English Usage, 3rd ed., § 366.2):

Need can also have the same present-tense forms as modal auxiliary
verbs ... In this case, need is normally followed by an infinitive
without to. She needn't reserve a seat - there'll be plenty of room.
These forms are used mainly in negative sentences (needn't), but they
are also possible in questions, after if and in other
'non-affirmative' structures. You needn't fill in a form.

Need I fill in a form?

I wonder if I need fill in a form.

This is the only form you need fill in. (BUT NOT You need fill in a form.)

So, it seems we can use need as a modal verb in an affirmative
sentence when a 'non-affirmative' word (such as only, hardly, seldom
etc.) gives the sentence a negative kind of meaning. Look at Swann's
last example: the sentence becomes incorrect when only is removed.

Andre Doubois · Answer

I think it is regional usage. For example, "this needs solved" is common in western Pennsylvania. It might show up elsewhere. It is not grammatically correct.

Samyar Esfandiari · Answer

I had red about "Need" and "Need to". When you use this sentence:
"you do not need paper," means it is not necessary.
When someone tell you: You don't need to bring any documents, mean it is not necessary and he/she has power to order you about someting

"need to do" vs "need do"

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