“The non-finite verb that has the uninflected form of the verb: be, say, dig, make. The form may be used alone (the BARE INFINITIVES I made him tell the truth). The bare infinitive is commonly used after a modal auxiliary verb (be after the modal may in We may be late) and after the auxiliary verb do (I did answer your letter, They do know the difference). It is also found in the complementation of a small number of main verbs such as have, let, make, see, and hear (I had Tom paint the fence; The soldiers let us pass; They need us to leave). In some instances, either type of infinitive may be used: Steven helped Susan (to) teach the children good manners; What Sidney did was (to) help Justin with his homework. The to- infinitive has a wider distribution as the verb in an infinitive construction: (1) It may be a subject (To meet you was a great pleasure), though a variant with postponed subject is more usual: It was a great pleasure to meet you. (2) It may be the object in various types of verb complementation: I hope to see Judith and Percy soon; I asked John and Joyce to come to my party; Jeffrey and Rosalind want me to be there. (3) It may be introduced by a wh word: Anton and Stella asked me what to advise their elder son. (4) It may function in various semantic classes of adverbial: To set the alarm, press four digits; He grew up to be a fine man; To be frank, the meeting was boring.”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.