Why verbs carry more weight here
Portuguese drops the subject pronoun whenever it can. Not sometimes, routinely. Which means the ending on the verb is often the only thing telling you who is doing the talking, and a missed ending is a missed sentence.
That is the case for putting verbs ahead of almost everything else in your study. The endings are the load-bearing part. Below: the three regular families, the stem changes that break them, and the handful of forms Portugal uses that Brazil has quietly retired.
Three families
Every Portuguese verb ends in -AR, -ER or -IR, and each family conjugates to its own pattern. The -AR group is much the largest and much the best behaved, which is why falar shows up in every textbook. The -ER and -IR groups look alike in the present and then part company in the past tenses, where most of the confusion lives.
| Person | -AR (Falar) | -ER (Comer) | -IR (Abrir) |
|---|---|---|---|
| Eu (I) | Falo | Como | Abro |
| Tu (You - informal) | Falas | Comes | Abres |
| Ele/Ela/Você (He/She/You) | Fala | Come | Abre |
| Nós (We) | Falamos | Comemos | Abrimos |
| Eles/Elas/Vocês (They/You Plural) | Falam | Comem | Abrem |
You cannot skip tu
Plenty of learners arrive having quietly ignored the tu forms, because Brazilian materials let them. Brazil largely uses você with third-person verbs and leaves the tu conjugations alone. In Portugal tu is the ordinary way friends, colleagues and family speak to each other, and dodging it is not an option.
The good news is the pattern is easy. Present tense, tu always ends in -s: falas, comes, abres. Past tense it takes -ste: falaste, comeste, abriste. That is nearly the whole story.
It is also worth watching in fiction. The moment a character switches from tu to third-person address, something has happened between them, and the author is usually not going to spell out what.
When the stem itself moves
Endings are only half of it. In a good number of verbs the root changes too, usually because the stress has landed somewhere new and the vowel has shifted to suit.
Sentir turns its e into an i for eu sinto. Dormir turns its o into a u for eu durmo. Neither is optional and neither is signposted.
Then there are open and closed vowels, which are invisible on the page and very audible out loud. In levar, the e of levas is open, close to English "eh". In levamos the stress has moved off it and it collapses into a schwa. Same letter, two different sounds, no spelling change to warn you.
Two past tenses, chosen by shape
The choice is about aspect, not about when. A photograph or a stretch of film.
Pretérito perfeito is the photograph. Finished, bounded. Ontem eu vi o filme.
Pretérito imperfeito is the film. Habits, states, background. Antigamente eu via muitos filmes.
Portugal also has a trick with the imperfeito that is worth learning early because you will use it daily. Eu quero um café is grammatical and lands like an order. Eu queria um café, literally "I wanted", is what people actually say at the counter. Pushing the request into the past softens it, and there is no real English equivalent.