The Portuguese infinitive is just the verb itself – unconjugated. These generally translate as “to run, to drink, to love, to do” ETC.
ar verb » Eu não quero falar. I don't want to speak.
er verb » A gente vai fazer uma festa. We're going to have a party.
ir verb » Você quer ir ao hotel? Do you want to go to the hotel?
Verbs placed immediately after prepositions have to be in the infinitive form. The most famous examples are with verbs GOSTAR, PRECISAR & ACABAR:
Gostar (not followed by a verb) ➔
Eu gosto de você.
» I like you.
Gostar (with verb after) ➔
Eu gosto de comer peixe.
» I like to eat fish.
Precisar (not followed by a verb) ➔
Eu preciso de você.
» I need you.
Examples of other verbs with another right after it and a preposition:
Precisar (with verb after) ➔
Eu preciso acordar.
» I need to wake up.
Acabar (not followed by a verb) ➔ Acaba sendo muito difícil.
Acaba sendo muito difícil
» It winds up being very difficult.
Acabar (with verb after) ➔
Eu acabei de chegar.
» I just arrived.
a = at, to Vamos começar a correr. Let's start to running.
de = of, from Vou parar de tomar café. I'm going to stop drinking coffee.
em = in, on Tenho dificuldade em dizer nao. I have difficulty saying no.
sem = without Eu estou falando sem parar. I'm talking non-stop.
até = until Ela vai comer até morrer. She's going to eat until death.
para = to, for Ela esta pronta para viajar. She's ready to travel.