Learn Hardcore Spanish (Latin America): En clase de geometría dibujamos un triángulo en la pizarra y pasamos diez minutos midiendo cada lado. - In geometry class we draw a triangle on the board and spend ten minutes measuring each side.

En clase de geometría sounds more general, like:
- In geometry class (in general, as an activity / as a subject)…
En la clase de geometría sounds more specific:
- In the geometry class (this particular session / this specific class)…

Spanish often drops the article after en clase de when we are talking about a type of class in a general or habitual sense:

En clase de inglés siempre hablamos mucho.
En clase de matemáticas usamos la calculadora.

So the version in the sentence suggests a general situation: what happens in geometry class as an activity, not necessarily in one specific day’s class.

Does dibujamos mean we draw or we drew here?

Dibujamos can mean either:

Present: we draw
Preterite (simple past): we drew

For -ar verbs, the nosotros form is the same in the present and in the preterite:

dibujamos = we draw / we drew
trabajamos = we work / we worked

So:

En clase de geometría dibujamos un triángulo…
could be:
- In geometry class we draw a triangle… (present, habitual)
- In geometry class we drew a triangle… (past, on a specific occasion)

Context usually makes it clear. If the rest of the text is in the past, you read it as we drew; if the context is describing a routine, it’s we draw.

Is pasamos also ambiguous like dibujamos (present vs past)?

Yes. Pasamos (from pasar) is also identical in:

Present: we spend → pasamos diez minutos… = we spend ten minutes…
Preterite: we spent → pasamos diez minutos… = we spent ten minutes…

So the whole sentence can be:

Present, habitual:
In geometry class we draw a triangle on the board and spend ten minutes measuring each side.
Past (a specific event):
In geometry class we drew a triangle on the board and spent ten minutes measuring each side.

Again, context outside this sentence would clarify which tense is intended.

Why is it un triángulo, not el triángulo?

Un triángulo is an indefinite triangle: just a triangle, not any specific one that was already known in the conversation.

un triángulo = a triangle (one triangle, not previously identified)
el triángulo = the triangle (a particular triangle we already know about or have mentioned)

In class activities, Spanish usually uses un when talking about something the students create or introduce for the first time:

Dibujamos un círculo.
Escribimos una carta.
Leemos un poema.

So un triángulo fits because it’s a new triangle they are drawing for the exercise.

Why en la pizarra and not en el pizarrón? Are they different?

Both words refer to a board in a classroom, but usage varies by region:

la pizarra
- Common in many countries and fully understood everywhere.
- Often used for chalkboards and now also for whiteboards.
el pizarrón
- Very common in much of Latin America (e.g., Mexico, parts of Central and South America).
- Also means classroom board.

In Latin America, you will hear both. The sentence uses la pizarra, which is neutral and widely understood. You could also say:

…dibujamos un triángulo en el pizarrón…

and it would be perfectly natural in many Latin American countries.

What does the structure pasar + time expression + gerund mean in pasamos diez minutos midiendo?

The pattern pasar + amount of time + gerund means to spend (time) doing something.

So:

pasamos diez minutos midiendo cada lado =
- we spend ten minutes measuring each side
  or
- we spent ten minutes measuring each side (if read as past)

More examples:

Pasé toda la tarde estudiando.
I spent the whole afternoon studying.
Pasamos dos horas hablando.
We spent two hours talking.

The gerund (midiendo, estudiando, hablando) expresses the ongoing action during that time.

Why is it midiendo and not mediendo or mediendo? How is midiendo formed?

Midiendo is the gerund (the -ing form) of medir (to measure).

For many -ir verbs that are stem-changing in the present (e→i or o→u), the stem change also appears in the gerund:

medir → present: mido, mides, mide…
→ gerund: midiendo (e → i)
pedir → present: pido, pides, pide…
→ gerund: pidiendo
dormir → present: duermo, duermes…
→ gerund: durmiendo (o → u)

That is why it’s midiendo, not mediendo. The e in medir changes to i.

Could I say pasamos midiendo cada lado durante diez minutos instead? Is that the same?

Yes, you could say:

Pasamos midiendo cada lado durante diez minutos.

This is very close in meaning to pasamos diez minutos midiendo cada lado. Both mean we spend ten minutes measuring each side.

The differences are tiny:

pasamos diez minutos midiendo…
- Emphasizes the amount of time first.
pasamos midiendo… durante diez minutos
- Emphasizes the activity first, then specifies the time.

Both are natural and correct. The original version is slightly more common in everyday speech when giving a specific length of time.

Why cada lado and not todos los lados?

Both are possible, but they sound slightly different:

cada lado = each side, focusing on the sides individually:
- You measure one side, then another, then another.
todos los lados = all the sides, focusing on the group as a whole:
- You measure the set of sides.

In many contexts, especially with actions repeated over parts of something, cada is very natural:

Lavé cada vaso a mano.
Revisamos cada respuesta.

Here, midiendo cada lado suggests they took the time to measure every individual side of the triangle one by one, which fits the idea of a careful geometry exercise.

Why doesn’t the sentence say nosotros? How do we know the subject is we?

In Spanish, subject pronouns (yo, tú, él, nosotros, etc.) are often dropped because the verb ending already shows who the subject is.

dibujamos = we draw / we drew
pasamos = we spend / we spent

The -mos ending clearly marks nosotros (we), so it’s not necessary to state nosotros unless you want to emphasize it.

Nosotros dibujamos un triángulo… (with emphasis on we, not someone else)
Dibujamos un triángulo… (neutral, default)

In everyday speech and writing, the shorter version without nosotros is more common when there’s no ambiguity.

Why en clase de geometría and not a clase de geometría?

The preposition depends on the meaning:

ir a clase = to go to class (movement, destination)
- Voy a clase de geometría. → I go to geometry class.
estar en clase = to be in class (location, situation)
- Estoy en clase de geometría. → I am in geometry class.

In your sentence, we are talking about what happens during or in geometry class, so Spanish uses en:

En clase de geometría dibujamos…
In geometry class, we draw…

Using a here would be incorrect because we are not describing movement toward the class, but an activity that takes place in that setting.

Why do triángulo and geometría have accents, but pizarra and clase do not?

Spanish accents mark the stressed syllable when it does not follow the default stress rules.

Basic rules:

Words ending in vowel, -n, or -s are normally stressed on the second-to-last syllable.
Words ending in other consonants are normally stressed on the last syllable.
If the stress is somewhere else, you usually need a written accent.

Now look at the words:

triángulo
- Ends in a vowel (o), so default stress would be on -gu- (tri-an-GU-lo).
- Actual stress is on -án- (tri-ÁN-gu-lo), not where the rule puts it → so it needs an accent: triángulo.
geometría
- Ends in a vowel (a), default stress would be on -tri- (geo-me-TRI-a).
- Actual stress is on -rí- (geo-me-TRÍ-a) → needs an accent: geometría.
pizarra
- Ends in a vowel (a), and the natural stress is on the second-to-last syllable: pi-ZA-rra.
- That matches the rule → no accent.
clase
- Ends in a vowel (e), stress naturally on the second-to-last: CLA-se.
- Also follows the rule → no accent.

So triángulo and geometría break the default pattern and therefore take accents; pizarra and clase follow the default and do not.