Portuguese kept two verb forms that its neighbours lost, and both of them are compulsory in ordinary speech rather than literary survivals. The personal infinitive takes a person ending while staying an infinitive, so antes de eles chegarem puts a subject inside an infinitive clause. The future subjunctive covers anything not yet real: quando eu chegar, se voce quiser, and there is no way to say those sentences without it.
A learner who postpones either of them cannot parse a large class of everyday sentences, which is why both belong near the beginning rather than in the chapter on advanced structures.
Six patterns that carry the grammar
- Marta é de Lisboa and Marta está em Lisboa. Two verbs for is: one for what something permanently is, one for where or how it currently is.
- estou a falar and estou falando, I am speaking, European and Brazilian respectively. Same meaning, two different constructions, and each sounds foreign in the other country.
- Falei ontem and Falava sempre, I spoke yesterday and I used to speak. Completed against habitual, the same split the rest of the family makes.
- Se eu tivesse tempo, if I had time. The if clause takes the imperfect subjunctive, which is alive and used constantly in ordinary speech.
- o, a, os, as, the definite articles, and Portuguese uses them where English does not: before names, before possessives, and before countries.
- por and para, both by or for. Por is cause and route; para is destination and purpose.
Two forms with nowhere else to send you
The personal infinitive exists because Portuguese needed a way to let a subordinate clause carry its own subject without forcing a subjunctive around it. Spanish has to say antes de que ellos lleguen with a conjugated subjunctive; Portuguese conjugates the infinitive instead and keeps the clause light. Once you have seen it the construction is obvious, and until you have seen it the sentence looks like an error.
The future subjunctive is the more surprising one, because Spanish has effectively lost it and nothing in a Spanish speaker's instinct predicts it. It appears after se, quando and enquanto whenever the event is still hypothetical, and its forms are built from the third person plural of the preterite, which is a reliable rule and one of very few in this area. Se eu for, quando ele fizer, se voces quiserem. Getting it wrong produces a sentence a Portuguese speaker will understand and immediately notice.
Everything else in the verb system is close enough to Spanish and to Italian to transfer. The endings rhyme, the tenses line up, and the irregulars are largely the same irregulars. That is the head start, and these two forms are where it stops.
Skipping the future subjunctive and saying the present instead
Here is the stall, and it is invisible because the substitute works. A learner who has not built the future subjunctive says quando eu chego instead of quando eu chegar, and is understood every time, because the meaning arrives from the context and the listener is being helpful. Nothing corrects it.
The habit then hardens for two reasons that reinforce each other. The form is only required in subordinate clauses, so a speaker who keeps their sentences short never meets the requirement, and the wrong version is grammatical Portuguese saying something slightly different, rather than a broken sentence anybody would stop for. The exit is mechanical: take the four or five conjunctions that demand it, write ten sentences with each, and say them aloud until the form arrives before the thought does. It is a fortnight, and it is the kind of thing worth taking to somebody who will interrupt, which is what a teacher is worth, and what you can do alone.
Where the head start runs out
The same sentence, and what each language has to do with it
Nine forms in ten transfer, which is what makes the tenth expensive: the guessing has already become automatic.
Only the last row is a variety question. The first four are the language itself, and a Spanish speaker who guesses through them will be understood every time and will be saying something slightly different in two of the five.
Two references for a system with two varieties
Priberam is the free dictionary and it does the one thing a bilingual dictionary will not: it marks which senses are Brazilian and which are European, and it gives full conjugation tables for any verb typed into it. It replaced the printed dictionary that had to pick a side. Use it as the check rather than the study, since reading a table is recognition and these endings have to be produced.
Conjuga-me is the second and it is narrower and faster: type any verb in any form and it returns the whole paradigm, including the personal infinitive and the future subjunctive laid out beside the rest instead of banished to an appendix. It replaced the verb book, which orders the forms by how a grammarian sees them rather than by how a learner meets them. Free.
The mistake: assuming Spanish will cover the endings
The verb systems rhyme closely enough that a Spanish speaker can guess most Portuguese forms and be right, and that is exactly what makes the misses expensive. The guessing works for perhaps nine forms in ten, so it becomes automatic, and the tenth is produced with the same confidence as the other nine.
The two forms above are the largest holes and they are not the only ones. The Portuguese preterite and imperfect divide time on the same line Spanish does, so that transfers, but the perfect does not: tenho falado in Portuguese means something closer to a repeated action over recent time, not the completed act that the Spanish he hablado reports. A Spanish speaker will use it as their own perfect for years and say something different from what they meant every time. The general shape of that trap, where a head start removes the pressure that would have corrected you, is the argument in the false friends Spanish speakers walk straight into.
What the endings still depend on
Every form above is unstressed at the end of a word, and how much of it survives into speech depends on which variety you are hearing, which is nasal vowels, and the sounds that separate the two coasts. Where the pronouns attach to these verbs is decided by the same choice, and it is a bigger difference than it looks, in sound, grammar and pronouns: what really differs. Producing any of it inside a conversation rather than on paper is getting to a conversation, in either variety. And storing a verb with its pair of unusual forms attached is a card design question, described in making cards that teach rather than cards that pile up.