En clase intento no bostezar, aunque a veces me acuesto tarde y tengo sueño.

En clase is a very common expression meaning in class or during class in a general sense.

• En clase = in class / during lessons
• En la clase = in the class, referring to a specific class or classroom

So here, En clase intento no bostezar means In class, I try not to yawn.

If you said en la clase, it would sound more specific, as if you were talking about one particular class.

Because after intentar, Spanish normally uses an infinitive.

• intento bostezar = I try to yawn
• intento no bostezar = I try not to yawn

You do not use a second conjugated verb here.

So:

• intento = I try
• bostezar = to yawn

This is the same pattern as:

• Quiero comer = I want to eat
• Prefiero dormir = I prefer to sleep
• Intento estudiar = I try to study

Because no is negating the action of yawning, not the verb intento.

• Intento no bostezar = I try not to yawn
• No intento bostezar = I don’t try to yawn

These mean different things.

So in the sentence, the speaker is still trying; what they are trying to avoid is bostezar.

Bostezar means to yawn.

It does not mean to be sleepy.
In this sentence:

• bostezar = the physical action of yawning
• tener sueño = to feel sleepy

So the idea is: I try not to yawn in class, although sometimes I go to bed late and I’m sleepy.

In Spain, the z in bostezar is usually pronounced like the th in thin.

So it sounds roughly like:

bos-te-THAR

A few pronunciation notes:

• bo as in a short bo
• ste
• zar with Spain Spanish th
• the final r is a light tapped r

In much of Latin America, that z would sound like an s instead.

Aunque means although / even though.

In this sentence:

• aunque a veces me acuesto tarde y tengo sueño
• although sometimes I go to bed late and I’m sleepy

It takes the indicative here because the speaker is talking about something real and factual.

Compare:

• Aunque me acuesto tarde, tengo que levantarme temprano.
  = Even though I go to bed late, I have to get up early.
  Real situation → indicative

Spanish can also use the subjunctive after aunque when the idea is uncertain, hypothetical, or not presented as a fact.

Acostarse means to go to bed, and it is a reflexive verb.

The reflexive pronoun changes with the subject:

• me acuesto = I go to bed
• te acuestas = you go to bed
• se acuesta = he/she goes to bed

It is reflexive because the subject is, in a sense, putting themself to bed.

Also, acostarse has a stem change in the present tense:

• acostar → acuesto
• acuestas
• acuesta

So me acuesto tarde means I go to bed late.

Because acostarse is a stem-changing verb in the present tense.

The o changes to ue in most present-tense forms:

• me acuesto
• te acuestas
• se acuesta
• nos acostamos
• os acostáis
• se acuestan

So acosto would be incorrect here.

This is the same kind of change you see in verbs like:

• dormir → duermo
• poder → puedo
• volver → vuelvo

Here, tarde is an adverb meaning late.

• me acuesto tarde = I go to bed late

You do not need a preposition like at or late in as you would in English. Spanish often uses a simple adverb here.

Compare:

• llego tarde = I arrive late
• cenamos tarde = we eat dinner late
• me dormí tarde = I fell asleep late

Because tener sueño is the normal Spanish expression for to be sleepy.

Literally, it is I have sleep, but idiomatically it means I’m sleepy.

So:

• tengo sueño = I’m sleepy
• estoy cansado/a = I’m tired

These are related, but not exactly the same:

• tener sueño = feeling sleepy, like you could fall asleep
• estar cansado/a = feeling tired or exhausted

A useful detail:

• tengo sueño = I’m sleepy
• tengo un sueño = I have a dream

So the article changes the meaning.

A veces means sometimes.

In the sentence, it modifies the second part:

• aunque a veces me acuesto tarde y tengo sueño
• although sometimes I go to bed late and I’m sleepy

Yes, it can often move around:

• A veces me acuesto tarde.
• Me acuesto tarde a veces.

The most natural position is often before the verb or near the start of the clause.

The comma helps separate the main idea from the contrasting idea introduced by aunque.

• En clase intento no bostezar, aunque a veces me acuesto tarde y tengo sueño.

This is very natural punctuation in Spanish. It marks a pause and makes the contrast clearer:

• main idea: I try not to yawn in class
• contrasting idea: although sometimes I go to bed late and I’m sleepy

In shorter sentences, punctuation can vary a little, but here the comma is completely normal.

Yes, but the meaning changes slightly.

• tengo sueño = I’m sleepy
• estoy cansado/a = I’m tired

If the reason for yawning is that you feel like sleeping, tengo sueño is the better choice.

If you just want to say you feel worn out, estoy cansado/a works.

So this sentence uses tengo sueño because it connects very naturally with bostezar.