Are all phrase verbs informal?

Shoe's and Edwin Ashworth's answers are useful, but one thing I think can be added is that "phrasal verb" is not a very specific term, so you may need to get more specific.

In particular, many of the examples of "formal phrasal verbs" in those two answers are actually verbs that take prepositional phrases as complements.

I don't think "prepositional verbs" sound informal

Verbs like "abide by", "accede to", "disabuse of", "engage in", "enlarge on/upon" all fall into that category: according to most analyses of English syntax, the second word here is a preposition that does not form a phrase with the preceding verb, but forms a prepositional phrase with the following noun phrase.

I would say prepositional verbs do not have any special tendency to sound informal. Many formal latinate verbs take a prepositional phrase as their complement; in fact, there are verbs in Latin itself that work the same way. For example, accedo, the Latin verb that is the source of the verb accede, could take as its complement a prepositional phrase starting with ad (the Latin preposition meaning "to").

One thing to consider: the highly formal "pied-piping" construction found in phrases like "important problems to which we must attend" is fully compatible with prepositional verbs like "attend to". If prepositional verbs were inherently informal, it would be odd if they could be used freely in such a formal construction.

"Particle verbs" might sound informal, but not always

The other type of "phrasal verb" is what are also called "particle verbs": words where the verb is accompanied by a word that (usually) has the same form as a preposition, but that does not take a noun phrase as its complement. For example, in the sentence "You need to push in the tab using a screwdriver", the word "in" does not form a phrase with the noun phrase "the tab", which can be seen from the alternative word order "You need to push the tab in using a screwdriver." Traditionally, words like in were considered to be used as "adverbs" in this kind of sentence, but modern linguists may categorize them as prepositions used intransitively.

Unlike prepositional verbs, particle verbs do not have a close parallel in classical Latin (Latin verbs could be prefixed with prepositions, but they couldn't have prepositions used without nouns as separate words going with the verb). This construction is I think the main type of "phrasal verb" that might be felt to be informal. "Carry out" is a particle verb: we can say "carry it out", showing that "out" does not form a phrase with a noun in this construction. I don't actually find that "carry out" sounds particularly informal to me, but I have never tried to write with a very high degree of formality (as you see, I use contractions, another feature that is sometimes said to be "informal"), so you'll have to look at the usage in the community that you want to be a part of, as Shoe advised.

This article from Typely.com amplifies and enlarges upon previous answers (there is a previous thread). It includes some useful examples (though the actual definition of 'phrasal verb' they use seems, from some claimed examples, more than usually imprecise), classifying by register (formality):

Because of their frequent occurrence in informal speech and writing, it’s not unreasonable to think that phrasal verbs are always informal. You may have read that phrasal verbs should be avoided in formal writing, and that the single verb equivalents should be used instead. While to some extent that’s true, the reality is more complicated.

Phrasal verbs in formal writing

There are many phrasal verbs that can and also should be used in formal writing. You will often see them used in many quite formal and formal texts such as business letters, academic writing, scientific papers, technical papers, legal documents, news reports, and official government documents.

Before we move on to examples of phrasal verbs that are useful and appropriate for formal writing, let’s look at language register for a moment...

‘Register’ is the term we use to refer to different varieties or styles of speaking and writing, and also the degree or level of formality with which we speak or write. Degree of formality is on a sliding scale rather than in distinct categories, and although phrasal verbs are often thought of as an informal part of language, most of them are neutral, and some are in fact rather formal.

Some phrasal verbs are definitely informal, for example:

• beaver away [at] – work hard for a long time
• belt out [T] – sing or play a musical instrument very loudly
• harp on [about] – talk non-stop about something in a boring or annoying way
• pig out – eat a lot of or too much food

In your formal writing, you should of course avoid phrasal verbs that are at the informal end of register, and steer clear of slang phrasal verbs, and those that would be considered by many to be offensive. A good phrasal verb dictionary will tell you which phrasal verbs are informal, slang, or offensive.

It's true that very often, single verbs are more formal and therefore are more appropriate for formal writing than their phrasal verb equivalents. Some examples of these are:

• Single verb/Phrasal Verb

• constitute/make up

• calculate/work out

• cause/bring about

• discover/find out

• discuss/talk about

• emerge/come out

• eradicate/stamp out

• increase/go up

• maintain/keep up

• organize/set up

• propose/put forward

• select/pick out

However, most phrasal verbs are neutral, neither informal or formal, and in general there’s no reason to specify they shouldn’t be used in formal writing. In fact, in some cases it’s more appropriate to use a phrasal verb in place of a single verb. For example, the phrasal verb ‘carry out’:

“Researchers carried out a survey into …” sounds much better for formal writing than

“Researchers did a survey into …”

At the other end of the formality register, there are phrasal verbs that are so formal they’re only used in very formal or serious speech or writing. If you were to use them in informal writing they would very strange and out of place.

Examples of formal phrasal verbs

• adhere to
• appertain to
• ascribe to
• disabuse of
• emanate from/to
• depart from
• engage in
• enlarge on/upon
• enter on/upon
• offend against
• permit of
• pertain to
• provide against
• set forth

As always with writing, context is everything – remember to use the language most appropriate to your audience. Your university, organization, etc. will most probably have a guide to the language you should use for formal writing, including their preferences for using phrasal verbs or their single verb equivalents.

Summary

Often, single verbs are more formal than phrasal verbs and therefore can be more appropriate for formal writing than their phrasal verb equivalents. Nevertheless, phrasal verbs are very common in formal writing. Offensive, slang, and informal phrasal verbs are not appropriate for formal writing. Most phrasal verbs are neutral and therefore, in general, there’s no reason to specify they shouldn’t be used in formal writing. Some phrasal verbs are so formal they’re only used in very formal or serious writing. Yes, you can use phrasal verbs in formal writing, as long as you choose those that are the most suitable for your context or audience.