The infinitive is the simple, unconjugated form of a verb. It does not indicate who is doing the action or any time reference. "To be" is a verb in the English infinitive form. "Ser" is a verb in the Spanish infinitive form. When you look up a verb in the dictionary it is found in the infinitive form.
In Spanish the infinitive forms always end with "ar", "er" or "ir". Examples: "amar" (to love), "temer" (to fear), "partir" (to leave).
The infinitive can be used for a lot of things in Spanish grammar. It functions as a gerund in Spanish. In the following examples, I will use "nadar" (to swim):
- Yo quiero "nadar".--- I want to swim.
- Yo pensaba que "nadar" era divertido---I used to think that swimming was fun