Al principio de la clase, me pongo nervioso, pero luego me siento más tranquilo.

Al principio is the normal Spanish way to say at the beginning in this kind of sentence.

• al = contraction of a + el
• principio = beginning

So al principio de la clase means at the beginning of class.

En el principio is much less common here and often sounds unnatural in everyday Spanish. It may appear in more special contexts, especially when talking about in the beginning in a broad or historical sense.

In Spanish, a + el normally contracts to al.

So:

• a + el principio → al principio

This is a standard rule.
The same happens with de + el → del.

Examples:

• Voy al médico.
• Vengo del trabajo.

One important exception: if el is part of a proper name, you do not contract it, as in a El Escorial.

Because clase is a feminine noun.

So:

• el principio
• la clase

That gives:

• al principio de la clase

If the noun after de is feminine, you use de la.
If it were masculine singular, you would often get del:

• al principio del día

Because ponerse often means to become or to get a certain state, especially an emotional or physical state.

So:

• me pongo nervioso = I get nervous / I become nervous
• estoy nervioso = I am nervous

In this sentence, me pongo nervioso highlights the change at the beginning of class. It suggests that the nervousness starts or appears at that moment.

Here ponerse means to become or to get.

It is used reflexively:

• me pongo
• te pones
• se pone

This is simply how the verb is used in Spanish when talking about changes in mood, emotion, or condition.

Common examples:

• ponerse triste = to get sad
• ponerse contento = to become happy
• ponerse enfermo = to get sick
• ponerse rojo = to turn red

So me pongo nervioso is a very natural way to say I get nervous.

No. Here sentirse does not mean to sit.

There are two different verbs:

• sentar / sentarse = to sit
• sentir / sentirse = to feel

So:

• me siento más tranquilo = I feel calmer

This is from sentirse, which is commonly used with adjectives to talk about how someone feels:

• me siento cansado = I feel tired
• me siento mejor = I feel better
• me siento tranquilo = I feel calm

They use two different verbs because they express two slightly different ideas:

• ponerse = to become / to get
• sentirse = to feel

So:

• me pongo nervioso = I become nervous
• me siento más tranquilo = I feel calmer

The first part focuses on the change into a state.
The second part focuses on the feeling itself.

This combination is very natural in Spanish.

Yes. These adjectives must agree with the person being described.

If the speaker is a man, you usually say:

• nervioso
• tranquilo

If the speaker is a woman, you would say:

• nerviosa
• tranquila

So a female speaker would say:

• Al principio de la clase, me pongo nerviosa, pero luego me siento más tranquila.

Más tranquilo means calmer or more calm.

The word más is used to make a comparison:

• tranquilo = calm
• más tranquilo = calmer

In this sentence, the idea is that later on, the speaker feels calmer than before.

Spanish often uses más + adjective where English might use an -er adjective:

• más tranquilo = calmer
• más feliz = happier
• más fácil = easier

Here luego means later / then.

In this sentence:

• pero luego = but then / but later

Yes, it is very similar to después in many contexts. Both can often mean later or afterwards.

Examples:

• Luego me siento mejor.
• Después me siento mejor.

Both are natural. Luego is very common in everyday Spanish.

The present tense is often used in Spanish to describe:

• habits
• routines
• things that generally happen

So this sentence suggests something like a regular experience:

• at the beginning of class, this happens
• later, this happens

It is not necessarily describing only one specific class. It can mean this is what usually happens to the speaker.

Yes, you could, but the meaning changes slightly.

• Estoy nervioso al principio de la clase = I am nervous at the beginning of class
• Me pongo nervioso al principio de la clase = I get nervous at the beginning of class

The version with ponerse emphasizes the moment the feeling appears.
The version with estar just describes the state.

Both are correct, but me pongo nervioso is a better match if you want to stress that the nervousness starts then.

Because Al principio de la clase is an introductory time phrase.

Spanish often uses a comma after an introductory phrase, especially when it helps readability:

• Al principio de la clase, me pongo nervioso...

You may sometimes see short introductory phrases without a comma in informal writing, but this comma is perfectly natural and clear.

It can depend on context.

La clase can mean:

• the class session
• the lesson
• the classroom class period

So al principio de la clase could mean:

• at the beginning of class
• at the beginning of the lesson

In normal learning contexts, class is usually a very good translation.

No, not in this sentence.

Both verbs here are being used reflexively:

• me pongo
• me siento

If you remove me, the sentence becomes incorrect or changes meaning completely.

For example:

• pongo by itself means I put
• siento by itself usually means I feel or I regret, but not in the same structure as sentirse

So the pronoun is necessary here.

Only the adjectives would change to feminine form:

• Al principio de la clase, me pongo nerviosa, pero luego me siento más tranquila.

The verbs stay the same:

• me pongo
• me siento

It is the describing words—nervioso/tranquilo—that must agree with the speaker.