Después de cenar, nos sentamos en la terraza y tomamos un té de menta mientras hablamos de la semana.

Because después de is followed by a noun or an infinitive when the subject stays the same.

• después de cenar = after having dinner / after dinner
• después de comer = after eating
• después de la cena = after dinner (using a noun instead)

If the subject changes, Spanish usually uses después de que + conjugated verb:

• Después de que ellos cenan/cenaron...

In Spanish, after a preposition such as de, a, sin, or para, a following verb normally goes in the infinitive.

So:

• después de cenar
• antes de salir
• sin hablar
• para estudiar

This is a very common pattern.

Nos sentamos comes from the verb sentarse, which means to sit down.

• me siento = I sit down
• te sientas = you sit down
• nos sentamos = we sit down / we sat down

The nos is the reflexive pronoun, meaning something like ourselves.

Without the pronoun, sentar usually means something different, such as:

• to seat someone: Sentó al niño en la silla
• to suit someone: Esa chaqueta te sienta bien

So nos sentamos is not optional here; it is part of the verb sentarse.

They can be either, because for regular -ar verbs, the nosotros form is spelled the same in the present and the preterite.

So:

• sentamos = we sit or we sat
• tomamos = we drink/have or we drank/had
• hablamos = we talk or we talked

Only the context tells you which one is meant.

That means this sentence is grammatically ambiguous on its own. In many contexts, it may sound like a habitual present. If you wanted a clearer past sentence, Spanish often uses:

• Después de cenar, nos sentamos en la terraza y tomamos un té de menta mientras hablábamos de la semana.

Because en shows location, while a usually shows movement toward a place.

• en la terraza = on/in the terrace, there on the terrace
• a la terraza = to the terrace

Compare:

• Vamos a la terraza = We go to the terrace.
• Estamos en la terraza = We are on the terrace.
• Nos sentamos en la terraza = We sit down on the terrace.

In Spain, terraza often means an outdoor sitting area such as a terrace, patio, balcony space, or outdoor seating area.

It does not always mean a roof terrace. The exact meaning depends on the context.

For example:

• at home: a terrace/balcony/patio area
• at a café or bar: the outdoor seating area

So here it most likely means an outdoor place where they sat and had tea.

Because té with an accent means tea, the noun.

Without the accent, te is the object pronoun meaning you.

• té = tea
• te = you

This kind of accent is used to distinguish words that are otherwise spelled the same.

Both are possible, but they are slightly different.

• tomamos un té de menta = we had a mint tea / a serving of mint tea
• tomamos té de menta = we drank mint tea in a more general sense

Using un makes it sound more like a specific drink or serving, which is very natural with drinks in Spanish:

• tomar un café
• tomar un té
• tomar una cerveza

Té de menta is the normal way to name the type or flavour of the tea: mint tea.

Spanish often uses de for this kind of label:

• helado de vainilla = vanilla ice cream
• zumo de naranja = orange juice
• té de menta = mint tea

Con menta would suggest tea with mint added to it, focusing more on the ingredient rather than the standard name of the drink.

Mientras means while, and it introduces an action happening at the same time as another action.

So:

• tomamos un té de menta mientras hablamos = we have mint tea while we talk

Because hablamos is also a nosotros -ar form, it has the same present/preterite ambiguity mentioned earlier. In a clearly past narrative, many speakers would prefer:

• mientras hablábamos

That imperfect form makes the simultaneous background action clearer.

With hablar, the most common preposition is de:

• hablar de algo = to talk about something

So hablamos de la semana is the most natural everyday wording.

Hablar sobre is also possible, but it often sounds a bit more formal, deliberate, or topic-focused.

Also, la semana here means something like the week / the events of the week. Spanish often uses the definite article where English might use our or no article at all.

Because Después de cenar is an introductory time phrase, and Spanish often separates that kind of opening phrase with a comma.

• Después de cenar, nos sentamos...

The comma helps readability and is very natural in writing. In short sentences, punctuation can sometimes vary a little, but here the comma is standard and clear.