Learn Hardcore Portuguese (Portugal): Eu vou mudar de roupa antes do jantar. - I am going to change clothes before dinner.

Mudar de + noun means “to switch to a different X,” so mudar de roupa = “change (into different) clothes.”
Mudar + direct object means “to change/alter that thing,” often someone else’s or a specific item:
- Vou mudar a roupa do bebé. = I’m going to change the baby’s clothes.
- Vou mudar a roupa da cama. = I’m going to change the bed linen.
  For your own clothes, say mudar de roupa.

Can I say trocar de roupa instead of mudar de roupa?

Yes. Trocar de roupa is very common and natural in Portugal, usually interchangeable with mudar de roupa. If there’s any nuance, trocar hints at “swapping,” but in this context they’re effectively synonyms.

To change someone else’s clothes you’d use a direct object: trocar a roupa ao bebé.

Do I need a reflexive pronoun, like mudar-se?

No. Mudar-se means “to move house/relocate.”

Vou mudar-me. = I’m going to move (house), not change clothes.
If you want a reflexive verb about clothing, use vestir-se (to get dressed) or despir-se (to undress):
Vou vestir-me antes do jantar.

Why is there a de after mudar here?

It’s the idiomatic pattern mudar de + noun when you change from one option to another. Other common collocations:

mudar de casa/emprego/ideia/opinião/canal/assunto

What’s the difference between antes do jantar and antes de jantar?

Both are correct; the nuance is slight:

antes do jantar = “before the dinner” (points to that mealtime/time slot).
antes de jantar = “before dining/eating dinner” (focuses on the activity).
In everyday speech they often mean the same thing.

What is do in antes do jantar?

Do is the contraction of de + o (of/from + the). Because jantar is masculine, you get:

de + o = do → antes do jantar
Other contractions: da (de + a), dos (de + os), das (de + as).
Examples: antes da reunião, antes dos exames, antes das férias.

Is the subject pronoun Eu necessary?

No. In European Portuguese the subject is often dropped when the verb form makes it clear.

Vou mudar de roupa antes do jantar. is fully natural.

Could I use the simple future mudarei?

Grammatically yes, but in everyday European Portuguese the periphrastic future (ir + infinitive) is far more common:

Natural: Vou mudar de roupa…
Formal/rare in speech: Mudarei de roupa…

Can I move the time phrase to the front?

Yes.

Antes do jantar, vou mudar de roupa.
When fronted, use a comma.

How do I pronounce the sentence in European Portuguese?

Approximate guide (EP):

Eu ≈ “ehw”
vou with ou like English “oh”: “voh”
mudar stress on -dar: “mu-DAR”; final r is a light tap
de ≈ “dǝ” (very short, like “duh”)
roupa ≈ “ROH-pǝ”
antes ≈ “AHN-tesh” (final s like “sh”)
do ≈ “du”
jantar ≈ “zhahn-TAR” (j like “zh”; nasal ã) Spoken fast, you’ll hear linking: antes do ≈ “AHN-tesh du”.

Why is roupa singular? Why not roupas?

In Portugal, roupa is usually a mass noun meaning “clothes” collectively, so singular is normal: a roupa suja, apanhar a roupa.
Plural roupas can appear when emphasizing individual garments, but mudar de roupa is the idiomatic expression.

How do I make it negative or a question?

Negative: place não before the verb: Não vou mudar de roupa antes do jantar.
Yes/no question: keep the same order and use rising intonation: Vais mudar de roupa antes do jantar? (to “tu”). Formal: Vai mudar de roupa antes do jantar?

How do I say “change clothes for (the occasion of) dinner,” not just “before dinner”?

Use para for purpose:

Vou mudar de roupa para o jantar.
You could also specify the clothing: Vou vestir o pijama depois do jantar.

How can I specify who is dining with antes de?

Use the personal infinitive:

Antes de jantarmos, vou mudar de roupa. (before we have dinner)
Antes de ele jantar, vou mudar de roupa. (before he has dinner)
Antes de jantarem, vou mudar de roupa. (before they have dinner)

When would I use antes que instead of antes de?

Use antes que + subjunctive to introduce a clause (often about something that may or may not happen):

Antes que o jantar acabe, vou mudar de roupa.
With a noun or verb in general, prefer antes de: antes do jantar, antes de jantar.

How do I say I’m in the middle of changing right now?

In Portugal, the progressive is estar a + infinitive:

Estou a mudar de roupa.
Avoid Brazilian-style estou mudando in European Portuguese.

How do I give an instruction like “Change clothes before dinner”?

Informal (tu): Muda de roupa antes do jantar.
Formal (o senhor/a senhora/você): Mude de roupa antes do jantar.
Plural (vocês): Mudem de roupa antes do jantar.

Is jantar always “dinner”? What about “supper”?

In Portugal, jantar is the main evening meal (“dinner”). Ceia is a later, usually lighter, late‑night meal/snack. So your sentence clearly refers to the evening dinner.