Learn Hardcore Portuguese (Portugal): Tu não deves exagerar com o café, ou vais ficar nervoso. - You shouldn’t overdo the coffee, or you will get nervous.

tu = informal, used with friends, family, people your age or younger, in relaxed situations.
você = can sound distant, cold, or even slightly rude in Portugal, especially if used with someone you know well. It’s not the neutral polite “you” like in Brazilian Portuguese.

So this sentence is clearly informal, probably said to a friend or family member.

If you changed it to você, the verb would also change:

Tu não deves exagerar com o café…
Você não deve exagerar com o café… (grammatically OK but can sound off in Portugal depending on context)

Why is it não deves and not something like não deve or não tens de?

Two things are going on:

Agreement with “tu”
- tu → deves (2nd person singular)
- ele/ela/você → deve (3rd person singular)
So with tu you must say tu não deves, not tu não deve.
Meaning of dever here
Dever in this context means “should / ought to”, giving advice, not a strong obligation:

Tu não deves exagerar com o café
= You shouldn’t overdo it with coffee (advice).

Compare with:

Tu não tens de exagerar com o café / Tu não tens que exagerar com o café
Literally: “You don’t have to overdo it with coffee” → this sounds odd; the natural version with não tens de would be about not being obliged to do something, which doesn’t fit well here.

So não deves is the natural way to give advice in this sentence.

What exactly does deves mean here? Is it “must” or “should”?

In this sentence, deves is best understood as “should” / “ought to”, i.e. a recommendation:

Tu não deves exagerar com o café
→ “You shouldn’t overdo it with coffee.”

Nuances:

deves = quite strong advice (you really should/shouldn’t)
devias (imperfect) = softer, more tentative:
- Tu não devias exagerar com o café. → “You really shouldn’t / You’d better not overdo it with coffee.”

If the speaker wanted a strong obligation like “must not”, they’d normally choose a different verb:

Tu não podes exagerar com o café. → You mustn’t / you’re not allowed to overdo it with coffee.

Why is it exagerar com o café? In English we don’t say “exaggerate with coffee”.

The verb exagerar in Portuguese is used more broadly than English “exaggerate”. Besides “to overstate,” it also means “to overdo (something)” or “to have too much of something”.

exagerar com o café
Literally: “to exaggerate with coffee”
Natural meaning: “to overdo it with coffee / to have too much coffee.”

Other similar uses:

Não exageres no sal. → Don’t overdo the salt.
Ele exagera com o trabalho. → He overdoes it with work.

So here it’s about excessive consumption, not about telling exaggerated stories about coffee.

Why is the preposition com used: exagerar com o café and not something else?

With exagerar, Portuguese can use different prepositions depending on the nuance:

exagerar com [algo]
Often used with things you consume or use excessively:
- exagerar com o café / com a bebida / com o açúcar
  → overdo it with coffee / alcohol / sugar
exagerar no [algo]
Often used with a more general category or quantity:
- exagerar no sal / no volume / na comida
  → overdo the salt / the volume / the food.

In this sentence, com o café is very idiomatic and totally natural in European Portuguese.
Exagerar no café is also understandable, but com o café is the more usual choice here.

What does vais ficar mean? Is this a future tense?

Yes, vais ficar is a very common way to express the near future in Portuguese:

ir (present) + infinitive → near future / going-to future

Here:

tu vais = you go / you are going
ficar = to become / to get / to stay

So vais ficar nervoso ≈ “you’re going to get nervous” / “you will become nervous”.

This form is usually more natural in speech than the synthetic future:

ficarás nervoso (grammatically fine, but sounds more formal / written in European Portuguese).
vais ficar nervoso sounds normal and conversational.

Why use ficar in vais ficar nervoso? What does ficar add here?

In this sentence, ficar means “to become / to get” (a change of state), not “to stay” or “to remain”:

ficar + adjective = to become / to get + adjective

Examples:

ficar nervoso → to get/become nervous
ficar doente → to get sick
ficar zangado → to get angry
ficar triste → to become sad

So vais ficar nervoso ≈ “you’re going to get nervous”—you’re not nervous now, but you will become nervous if you drink too much coffee.

Why is it nervoso and not another form like nervosa or nervosos?

Adjectives in Portuguese agree with the gender and number of the noun or pronoun they describe.

Here the subject is tu, referring to one male person (implied by nervoso):

tu (masculine singular) → nervoso (masculine singular)

Other possibilities:

Speaking to one female:
- Tu não deves exagerar com o café, ou vais ficar nervosa.
Speaking to more than one person (mixed or all male):
- Vocês não devem exagerar com o café, ou vão ficar nervosos.
Speaking to more than one female:
- Vocês não devem exagerar com o café, ou vão ficar nervosas.

So the form of nervoso changes with the person’s gender and number.

Could we use senão instead of ou here? What’s the difference?

Yes, you could say:

Tu não deves exagerar com o café, senão vais ficar nervoso.

Difference:

ou literally means “or”.
senão in this context means “otherwise / or else”.

In practice, in sentences like this:

…, ou vais ficar nervoso.
…, senão vais ficar nervoso.

both convey: “If you do, you’ll get nervous.”

Senão makes the “or else / otherwise” idea a bit clearer. Using ou this way is very common and natural in speech.

Is the comma before ou necessary: …, ou vais ficar nervoso?

The comma is optional here and can reflect a slight pause in speech.

Tu não deves exagerar com o café, ou vais ficar nervoso.
Tu não deves exagerar com o café ou vais ficar nervoso.

Both are acceptable.

Writers often add the comma to mark the pause and the logical separation between:

The advice: Tu não deves exagerar com o café
The consequence: (ou) vais ficar nervoso

In spoken language, the pause is usually there whether or not you write the comma.