When do you use the future subjunctive in Portuguese?
Use it after quando, se, assim que, enquanto and depois que when talking about the future. Portuguese kept this tense in daily use where Spanish reduced it to legal formulas, so a Spanish speaker's instinct to use the present here is wrong: quando eu chegar, not quando eu chego.
The future needs the future subjunctive
This is one of the most common errors for Spanish speakers, because Spanish uses the present subjunctive in these clauses. Portuguese uses a dedicated future subjunctive, and it is obligatory rather than stylistic.
Quando eu chegar, ligo para ti.
When I arrive, I'll call you.
Se você quiser, podemos ir.
If you want, we can go.
Assim que eles chegarem, começamos.
As soon as they arrive, we'll start.
Built from the third-person plural preterite
Take the eles form of the simple past, drop the -am, and add the personal endings. For regular verbs the result is identical to the personal infinitive; for irregulars it is not, which is where the forms become worth learning.
falaram → falar, falares, falar, falarmos, falarem
regular — same as the personal infinitive
fizeram → fizer, fizeres, fizer…
fazer, irregular
foram → for, fores, for…
ser and ir share this
tiveram → tiver, tiveres, tiver…
ter, irregular
Present for habit, future subjunctive for the future
The same conjunction takes different moods depending on whether you mean a repeated fact or a specific future event. This distinction is real and audible.
Quando chego, tomo café.
When I arrive, I have coffee.
Habit — indicative.
Quando eu chegar, tomo café.
When I arrive (later today), I'll have coffee.
Specific future.
Common mistakes
Quando eu chego amanhã…
Quando eu chegar amanhã…
A specific future event after quando requires the future subjunctive.
Se você quer, podemos ir. (meaning: if you should want to)
Se você quiser, podemos ir.
A hypothetical future condition takes the future subjunctive; se with the present indicative states a current fact.
Quando eu for chegar…
Quando eu chegar…
The future subjunctive already carries the future meaning; no auxiliary is needed.
Frequently asked questions
Why does Portuguese have this when Spanish does not?
Both inherited it from Latin, and Spanish largely abandoned it — it survives there in fixed legal and proverbial phrases. Portuguese kept it in ordinary daily speech, which is one of the clearest grammatical divides between the two languages.
Is it the same in Brazil and Portugal?
Yes. This is one of the areas where the two varieties agree completely — the future subjunctive is fully alive in both.
How do I tell it from the personal infinitive?
For regular verbs you cannot from the form alone, and it rarely matters because the context settles the meaning. For irregular verbs they diverge clearly: for against ser, fizer against fazer.