The infinitive is the base form of a verb. In English, the infinitive is marked by placing a "to" before the verb, for example:
drink - this is just a regular verb
to drink - this is the infinitive of the verb
To conjugate a verb means to change its form to express tense, mood, person and so on. Verbs change as they are used with different people and different time.
With people:
I drink
You drink
He/She/It drinks
We drink
You drink
They drink
With time:
I drank
You have drunk
He/She/It had drunk
We/You/They will drink
In Spanish (tho you may not need this), the infinitive is marked by three endings + the stem of the verb:
