An infinitive is the basic form of a verb, usually appearing with the word “to” in front of it: to run, to think, to be, to understand. It's called the base form because you haven't added anything to show who is doing it or when it happened.

When you look up a verb in the dictionary, you find its infinitive form. The entry for “swim” shows to swim, not “swimming” or “swam” or “swims.” From this basic form, you can create all the other forms you need: “I swim,” “she swims,” “they swam,” “we were swimming.”

Infinitives appear throughout your writing and speech. You might say “I want to learn Spanish” or “She hopes to become a doctor.” Sometimes the “to” disappears but the infinitive remains, like when you say “I can speak French” or “Let me help you” (rather than “to speak” or “to help”).

Until you attach an infinitive to a subject and a time, the action stays undefined. To jump could mean anyone jumping at any time. Once you say “Maria jumped yesterday,” you've defined it. Understanding infinitives helps you see how language works at its most basic level.