The infinitive, is word you will often hear in your Spanish classes (especially when studying grammar) and so it’s very important that you know what it is!
The infinitive is the basic form of a verb. In Spanish, infinitives always end in –ar, -er, -ir. In English, the infinitive usually refers to the “to + verb” form of the verb such as “to cook” (cocinar) or “to paint” (pintar). Alone the infinitive doesn’t indicate a tense (when the action is taking place) or who is performing the action of the verb ad to do so it must be conjugated.
The infinitive form of the verb is (with most verbs) used as the stem for their conjugations. To conjugate regular verbs, you simply remove the infinitive ending ( -ar, -er, -ir) to find the stem, and then add the tenses ending.
Buscar (to look for) > -ar > Stem = Busc
Comer ( To eat) > -er > Stem = Com
Vivir (To live) > -it > Stem= Viv