To fly, wings must creat more lift. In what sense "to fly"is used in this sentence. Whether it is used as an adverb, an adjective or a noun

  • 2
    Compare To live, you must eat, a "stylised literary inversion" which "fronts" what I'd call an "adverbial" element - the default sequence in English being You must eat to live. In which context, to eat is an "adverb of purpose", more fully expressed as You must eat in order to live (less commonly, ...so as to live, but that version doesn't "front" so well in many contexts). Nov 19, 2021 at 17:06
  • Yes, in traditional grammar it would probably be regarded as adverbial (see "adverbial infinitive" here - webapps.towson.edu/ows/verbals.html ). In modern grammar the term "adjunct" is used (for the function within the sentence (as described by James K, while the part of speech is obviously a verb.
    – rjpond
    Nov 19, 2021 at 17:49
  • @rjpond: I think "free adjunct" as referenced in this stellar answer is an invaluable term to help people understand usages like Being jealous, Mona would not let her boyfriend dance with any of the cheerleaders. Where to my mind, being jealous and jealously could both be "correctly" labelled "adverbial elements", but calling one of them a "free adjunct" does seem useful. Not sure how useful "adjunct" is to OP here though. Nov 19, 2021 at 18:37
  • 2
    If you can add in order in front of the infinitive without changing the meaning, then it's what's called a Purpose Infinitive, because it expresses the purpose of the following sentence. This construction is often used for giving directions: To open, press lever. Nov 19, 2021 at 19:55
  • Yes, you can also put the to + infinitive at the end and see if it works: Wings must create more lift to fly. [more is slightly odd. Where is comparison?] Places to go, people to see, work to do. Very common in English.
    – Lambie
    Nov 19, 2021 at 21:12

3 Answers 3


None of the above, "To fly" is an infinitive verb. It's not a noun, adverb or adjective. It is a short, non-finite adjunct clause.


It’s equivalent to

In order to fly, wings must create more lift.

The clause states the purpose for which the wings must create lift. It modifies the verb “must create,” making it an adverbial clause.


There are multiple ways to parse this. James says none of the above, but I'd say that it functions as an adverb, modifying the verb phrase "must create". (This is similar to what Davislor says, except that I'd call "to fly" an infinitive phrase instead of a clause. As I said, you can parse this in multiple ways.)

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