The English infinitive is the basic form of the verb. It has two parts, the particle to and the present form of the verb. Here are some infinitives:
The infinitive is used with and without the particle to.
Used with the to, it’s called “the full infinitive” or “the to-infinitive.” The infinitive functions as different parts of speech:
Jenny wants to go to the circus. (noun, direct object of the verb “wants”)
To err is human. (noun, subject of verb “is”)
Charlie’s obsession is to win the lottery. (noun, complement of the verb “is”)
I’ll have two hamburgers to go. (adjective, modifies “hamburgers”)
She closed the door to keep the flies out. (adverb, tells why she closed the door)
Used without the particle, it’s called the “zero infinitive” or “the bare infinitive.”
One use of the bare infinitive is to form a main verb with a helping verb:
I should go home, but I’m embarrassed. (used with auxiliary “should” to form complete verb)
You might tell me when you are going to be late. (used with auxiliary “might” to form complete verb)
Sometimes either form will produce the same meaning, although the bare infinitive will sound less formal:
Will you have time to help me to do my algebra homework?
Will you have time to help me do my algebra homework?
Deciding whether to use the full infinitive or the bare infinitive is a common difficulty for ESL learners. Because the full infinitive is more common, a good rule to follow is to try the full infinitive first.
It’s also a good idea to memorize the list of modal verbs that are always followed by the bare infinitive: can, could, may, might, must, shall, should, will, and would.