Let's take the verb to think, in Portuguese pensar

In English you say I think, you think, he/she/it thinks, we think, you (plural) think, they think

It practically doesn't change, in Portuguese, however, we say 

Eu penso, tu pensas, ele/ela/você pensa, nós pensamos, vós pensais, eles/vocês pensam

Note: in spoken Portuguese it's common to hear the pronoun tu with the conjugation of você, but grammatically it isn't right

The verb changes depending on it's referring to, and if it's singular or plural, if it's past, present or future (actually it's more complicated than that, since we have more than a way to express past, present and future). We call that conjugations.

Past form

Eu pensei, tu pensaste, ele pensou, nós pensamos, vós pensastes, eles pensaram

Future form

Eu pensarei, tu pensarás, ele pensará, nos pensaremos, vós pensareis, eles pensarão

The ending of the verb changes and this ending indicates who we're talking about and the time what we're talking about happened, happens, is happening or will happen Eu pensarei-I'll think about it

Hello,

It's like verb tenses..

look here: https://www.conjugacao.com.br/

Write a verb and see the correct conjugation of it in portuguese.