En geometría, hoy hemos dibujado un triángulo, un círculo y un cuadrado en el cuaderno.

In Spain Spanish, hoy hemos dibujado is very natural because hoy refers to a time period that is still connected to the present.

• hemos dibujado = we have drawn
• dibujamos = we drew

In many parts of Spain, the present perfect is commonly used for actions done today, this week, this month, etc.

So:

• Hoy hemos dibujado... = natural in Spain
• Hoy dibujamos... may sound less natural in this context in Spain, though it is understandable

In much of Latin America, hoy dibujamos or hoy dibujamos / hoy dibujamos un...-type past usage is often more common.

Hemos dibujado is the present perfect.

It is formed with:

• hemos = first person plural of haber
• dibujado = past participle of dibujar

So:

• he dibujado = I have drawn
• has dibujado = you have drawn
• ha dibujado = he/she has drawn
• hemos dibujado = we have drawn

The subject is understood from the verb ending, so Spanish does not need nosotros here.

Spanish often omits subject pronouns because the verb ending already shows who is doing the action.

• hemos already means we have
• so (nosotros) hemos dibujado and hemos dibujado both mean we have drawn

Including nosotros is possible, but it usually adds emphasis or contrast:

• Nosotros hemos dibujado, pero ellos no.
  • We have drawn, but they have not.

In your sentence, no emphasis is needed, so leaving it out is normal.

En geometría means in geometry or in geometry class.

Here it gives the context:

• En geometría = in geometry / during geometry class
• hoy hemos dibujado... = today we have drawn...

It is a natural way in Spanish to say what subject or class something happened in.

You could think of it as:

• In geometry, today we have drawn...

The comma separates the introductory phrase En geometría from the rest of the sentence.

This is similar to English:

• In geometry, today we drew...

It helps signal that En geometría is setting the scene or topic first.
The sentence could sometimes appear without that comma in less careful writing, but with the comma it is clear and natural.

Hoy adds a specific time reference: today.

The verb hemos dibujado tells you the action happened in a time period connected to the present, but hoy tells you exactly which period.

So:

• Hemos dibujado un triángulo... = We have drawn a triangle...
• Hoy hemos dibujado un triángulo... = Today we have drawn a triangle...

It is not redundant; it gives useful information.

Spanish often repeats the article with each noun in a list, especially when each item is being presented individually.

So:

• un triángulo, un círculo y un cuadrado

This is very natural and clear.

Could you sometimes omit repetition in Spanish? Yes, in some contexts, but here repeating un sounds best and most standard because the sentence is naming separate objects.

Yes. All three nouns are masculine singular nouns, so they take un:

• el triángulo → un triángulo
• el círculo → un círculo
• el cuadrado → un cuadrado

Even though shapes do not have natural gender, in Spanish every noun has a grammatical gender.

Because in the present perfect with haber, the past participle does not agree with the object.

So you say:

• hemos dibujado un triángulo
• hemos dibujado dos figuras
• hemos dibujado líneas

The participle stays dibujado.

This is different from adjectives, which do agree:

• un cuadrado dibujado
• unas figuras dibujadas

But in hemos dibujado, it is part of the verb tense, not an adjective.

Spanish normally uses the definite article here:

• en el cuaderno = in the notebook

This sounds natural when referring to the notebook being used in class.

Also, en + el does not stay separate. It contracts:

• en el → en el (this one stays as written, unlike a + el = al; note that en el does not contract)
• so the phrase is simply en el cuaderno

You need el because cuaderno is a masculine singular noun:

• el cuaderno

Yes, nouns ending in -o are often masculine, so cuaderno is masculine:

• el cuaderno
• un cuaderno

This is a very common pattern, though not every noun follows it.

Yes. Spanish word order is flexible, though some versions sound more natural than others.

Your sentence:

• En geometría, hoy hemos dibujado un triángulo, un círculo y un cuadrado en el cuaderno.

Other possible orders:

• Hoy, en geometría, hemos dibujado un triángulo, un círculo y un cuadrado en el cuaderno.
• Hoy hemos dibujado en el cuaderno un triángulo, un círculo y un cuadrado en geometría.
  This is grammatical, but less natural because en geometría works better near the beginning.

The original sentence is good because it introduces:

1. the class context
2. the time
3. the action
4. the objects
5. the place

The accent marks show which syllable is stressed.

• geometría → stress on -rí-
• triángulo → stress on tri-ÁN-
• círculo → stress on CÍR-

They are important in correct spelling and pronunciation.

Without the accents, the pronunciation rules would suggest different stress patterns, so the written accents are necessary.

Usually in the notebook is the safest translation.

But depending on context, cuaderno can also mean:

• notebook
• exercise book
• school notebook

In a classroom sentence like this, it refers to the notebook students write or draw in.

Yes. En el cuaderno tells you where the drawing happened: in the notebook.

It modifies the action hemos dibujado:

• we have drawn ... in the notebook

So the sentence means the shapes were drawn there, rather than, for example, on the board or on a worksheet.