En clase de matemáticas, la profesora escribió un número par y otro impar en la pizarra.

En clase de matemáticas is a very natural Spanish way to mean in maths class or during maths class.

• en clase de... = in ... class
• durante la clase de... = during the ... class

Both are possible, but en clase de matemáticas sounds more everyday and idiomatic here.

Examples:

• En clase de historia, vimos un documental.
• En clase de inglés, hablamos mucho.

So in this sentence, it sets the scene: In maths class...

After clase de, Spanish usually uses the subject name without the article.

So:

• clase de matemáticas
• clase de inglés
• clase de ciencias

Using de las matemáticas here would sound unnatural in normal Spanish.

This is similar to how Spanish often drops the article after a noun + de when naming a category or subject.

Spanish very often uses the definite article with professions or roles when talking about a specific person.

So:

• la profesora = the teacher
• el médico = the doctor
• la directora = the headteacher/principal

In English, we might sometimes say Teacher wrote... in a school context, but Spanish normally prefers la profesora if you mean a specific teacher already identifiable in the situation.

Escribió is the preterite of escribir.

• escribió = she wrote
• infinitive: escribir = to write

The preterite is used because this is a completed action in the past: the teacher wrote the numbers on the board, and that action is seen as finished.

Compare:

• La profesora escribió un número. = The teacher wrote a number.
• La profesora escribía un número. = The teacher was writing a number / used to write a number.

Here, escribió is the natural choice because it refers to one completed event.

By itself, escribió can mean he wrote, she wrote, or even you (formal) wrote, depending on context.

The reason we know it means she wrote here is because of la profesora, which is feminine.

So:

• la profesora escribió = the female teacher wrote
• el profesor escribió = the male teacher wrote

Spanish verb forms often do not show gender. The subject tells you that.

Par means even in maths.

• un número par = an even number
• un número impar = an odd number

In Spanish, adjectives often come after the noun, especially when they classify something in a straightforward way. Here, par and impar are classifying the type of number.

So Spanish says:

• número par
• número impar

not usually:

• par número
• impar número

Because otro normally does not take un before it in standard Spanish.

So Spanish says:

• otro libro = another book
• otra idea = another idea
• otro impar = another odd one / another odd number

Not:

• un otro libro
• un otro impar

In this sentence, otro stands in for otro número. Spanish often leaves out the repeated noun when it is obvious.

So:

• un número par y otro impar literally means:
• an even number and another odd one or more naturally in English:
• an even number and an odd number

Yes, exactly.

The full version would be:

• un número par y otro número impar

But Spanish often avoids repeating a noun when it is already clear. So número is omitted the second time.

That makes:

• un número par y otro impar

This is very natural Spanish.

A similar pattern:

• Compré un bolígrafo azul y otro rojo. = I bought a blue pen and another red one.

Because they express two different ideas:

• En clase de matemáticas = the setting or situation: in maths class
• en la pizarra = the physical location where she wrote: on the board

So the sentence gives both:

1. when/where socially the event happened: in maths class
2. where physically the numbers appeared: on the board

This is completely normal in Spanish.

Yes. In Spain, la pizarra is the normal word for board in a classroom.

Depending on context, it can refer to:

• a traditional blackboard
• a whiteboard

In many parts of Latin America, you may also hear el pizarrón.

So for Spanish from Spain, la pizarra is exactly what you would expect.

Yes. In maths, impar means odd.

But outside maths, impar can also mean something like unequal or unmatched, depending on context, though that is less basic and less common for beginners.

In this sentence, because it says número, the meaning is definitely mathematical:

• número par = even number
• número impar = odd number

Yes, Spanish word order is more flexible than English, but this sentence uses a very normal and clear order:

• En clase de matemáticas, = setting
• la profesora = subject
• escribió = verb
• un número par y otro impar = object
• en la pizarra = location

You could move some parts for emphasis, for example:

• La profesora escribió en la pizarra un número par y otro impar.
• En la pizarra, la profesora escribió un número par y otro impar.

These are possible, but the original sentence is very natural and neutral.

Yes, several:

• matemáticas
• escribió
• número

These accent marks matter for pronunciation and sometimes for meaning.

For example:

• escribio would be incorrect
• escribió shows the stress is on the last syllable: es-cri-bi-ó

Likewise:

• número is stressed on nú
• matemáticas is stressed on má

So they are important to learn exactly as written.