Learn Hardcore Portuguese (Portugal): Depois visitámos um aquário onde a guia explicou‑nos os hábitos dos peixes. - Afterwards we visited an aquarium where the guide explained the habits of the fish to us.

visitámos (with accent) = past tense (pretérito perfeito), we visited.
visitamos (no accent) = present tense, we visit / we are visiting.

The accent on á shows that the stress falls clearly on that syllable and also marks the past tense in the 1st person plural for many regular verbs in European Portuguese:
falámos (we spoke) vs falamos (we speak), cantámos vs cantamos, etc.

So in the sentence, visitámos must be past: we visited.

What tense are visitámos and explicou?

Both are in the pretérito perfeito (simple past):

visitámos = we visited
explicou = she/he explained

This tense talks about completed actions in the past, often in sequence. Here, first visitámos um aquário, then a guia explicou‑nos os hábitos dos peixes.

Why is it explicou‑nos and not nos explicou?

In European Portuguese, clitic pronouns like nos, me, lhe usually go after the verb (with a hyphen) in:

affirmative main clauses: Ela explicou‑nos tudo.
after some conjunctions, infinitives, and imperatives, etc.

Nos explicou is typical of Brazilian Portuguese word order. In Portugal, in a normal affirmative sentence, you say:

A guia explicou‑nos os hábitos dos peixes.
not
A guia nos explicou os hábitos dos peixes. (Brazilian usage)

What does nos mean in explicou‑nos? Is it a direct or indirect object?

Nos means to us / for us.

Grammatically, here it is an indirect object pronoun:

a guia = subject (the guide)
explicou = verb (explained)
nos = indirect object (to us)
os hábitos dos peixes = direct object (the habits of the fish)

So the structure is roughly: the guide explained *to us the habits of the fish*.

Why is there a hyphen in explicou‑nos?

When unstressed object pronouns (like me, te, nos, vos, o, a, lhe, etc.) come after the verb in Portuguese, they are joined with a hyphen:

explicou‑nos (explained to us)
mostrou‑me (showed me)
contaram‑lhe (they told him/her)

So the hyphen is just the normal way to write a clitic pronoun attached to a verb.

Why is it um aquário but os hábitos dos peixes?

This is about definite vs. indefinite articles:

um aquário = an / some aquarium (indefinite)
It introduces the aquarium as new information in the story: we visited *an aquarium*.
os hábitos dos peixes = the habits of the fish (definite)
We are now talking about specific, known habits: the habits that the guide is explaining. So Portuguese uses the definite article os.

So the pattern is:

new place: um aquário
specific thing explained there: os hábitos dos peixes

What exactly is dos in dos peixes?

Dos is a contraction of:

de + os = dos

It roughly means of the.

So:

os hábitos dos peixes = the habits *of the fish*

These contractions are very common:

do = de + o
da = de + a
dos = de + os
das = de + as

Why is it onde a guia explicou‑nos os hábitos dos peixes? Can I use something else instead of onde?

Onde is a relative pronoun that refers to a place (here, um aquário). It means roughly where.

So:

um aquário onde a guia explicou‑nos…
= an aquarium where the guide explained to us…

You could also, more formally, say em que or no qual instead of onde:

um aquário em que a guia explicou‑nos os hábitos dos peixes
um aquário no qual a guia explicou‑nos os hábitos dos peixes

But onde is the most natural in everyday language when the antecedent is a place.

Why is it a guia and not o guia?

The noun guia is common gender: it can be masculine or feminine, depending on the article:

o guia = the (male) guide
a guia = the (female) guide

In the sentence, a guia tells you the guide is female.

Why are peixes (fish) and hábitos (habits) plural? Could it be singular?

Os hábitos dos peixes is plural because:

There are many fish in an aquarium: peixes
Each species or group can have various habits: hábitos

Portuguese often uses the plural to talk about general characteristics of a group:

os costumes das pessoas – people’s customs
os hábitos dos peixes – fish’s habits

You could say o hábito do peixe, but that would sound like:

one specific habit
of one specific fish (or one general “fish” you’re focusing on)

The plural sounds more natural for a general explanation in this context.

Where can depois go in the sentence? Is the current position the only option?

Depois is an adverb meaning afterwards / then and is quite flexible in position. Natural options include:

Depois visitámos um aquário… (as in the sentence)
Visitámos um aquário depois… (less common here; usually followed by de
- verb/phrase)
Visitámos depois um aquário… (possible, but less neutral; it can sound more marked in style)

The most neutral and common in narrative is exactly what you see:

Depois visitámos um aquário…
with depois at the beginning, setting the time sequence.

Could we remove onde and make two separate sentences instead?

Yes, but the relationship between the events would change slightly.

Original:

Depois visitámos um aquário onde a guia explicou‑nos os hábitos dos peixes.
→ Emphasises that in that aquarium the guide gave the explanation.

Separate sentences:

Depois visitámos um aquário. A guia explicou‑nos os hábitos dos peixes.
→ The events are just sequential; it’s less explicit that the explanation happened in that aquarium. It could, from context, but the link is weaker.

The relative clause with onde ties the explanation directly to the aquarium as a place.

How should I pronounce visitámos differently from visitamos?

In European Portuguese:

visitámos (past): the stress is clearly on tá – vi‑zi‑*TÁ‑mos*.
visitamos (present): the stress is more naturally on ta but with a lighter, shorter vowel – vi‑zi‑*TA‑mos, and the final *‑mos is usually quite reduced.

The written accent in visitámos forces the stress and helps you recognise it as past tense.