A veces me siento en ese sillón al final del día y leo un rato para relajarme.

Because the verb here is sentarse, which means to sit down or to seat oneself. It is a reflexive verb, so it normally uses a reflexive pronoun:

• me siento = I sit down
• te sientas = you sit down
• se sienta = he/she sits down

Without me, siento usually comes from sentir and means I feel or I regret.

This creates a useful coincidence:

• me siento cansado = I feel tired
• me siento en ese sillón = I sit down in that armchair

So context tells you which verb is meant.

Because sentarse is a stem-changing verb in the present tense. The e in the stem changes to ie in most forms:

• me siento
• te sientas
• se sienta
• nos sentamos
• os sentáis
• se sientan

So me siento is the correct first-person singular present form.

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

• me siento already tells you I sit down
• leo already tells you I read

So yo is unnecessary unless you want emphasis or contrast:

• Yo leo, pero mi hermano ve la tele.

In your sentence, leaving out yo sounds natural.

In this sentence, me siento most naturally means I sit down or I sit myself down.

That is different from describing the state of already being seated:

• Me siento en ese sillón = I sit down in that armchair
• Estoy sentado en ese sillón = I am sitting in that armchair / I am seated in that armchair

So sentarse focuses more on the action of taking a seat.

A veces means sometimes.

It is often placed at the beginning of a sentence because it sets the time/frequency frame for the whole statement:

• A veces me siento...
• Me siento a veces...

Both are possible, but A veces at the front is very common and natural.

Also note that a veces is a fixed expression. It does not mean literally to times in modern usage; you should learn it as a chunk meaning sometimes.

Here, en means in or on, depending on what sounds natural in English. With seats, Spanish commonly uses en:

• sentarse en una silla
• sentarse en el sofá
• sentarse en ese sillón

In English we often say sit in an armchair or sit on a chair, but Spanish generally keeps en.

Silla means chair.

Sillón usually means a large, comfortable chair, often an armchair or easy chair.

So in this sentence, ese sillón suggests a specific comfortable seat, which fits well with reading and relaxing at the end of the day.

Spanish demonstratives depend on distance or perspective:

• este = this, near the speaker
• ese = that, near the listener or not especially close to the speaker
• aquel = that over there, farther away

So ese sillón means that armchair. It refers to a particular one that is identifiable in context.

In real life, the exact choice between este and ese can be a little flexible, but the basic idea is:

• este = this one
• ese = that one
• aquel = that one over there

Because a + el contracts to al in Spanish.

So:

• a + el final → al final

This contraction is mandatory, except when el is part of a proper name.

Examples:

• Voy al parque.
• Llego al final de la calle.

So al final del día is simply the correct contracted form of a el final del día.

Here it means at the end of the day or by the end of the day, in a literal time sense.

It refers to the last part of the day, when the speaker is winding down.

Be careful: in English, at the end of the day can also be idiomatic, meaning when everything is considered. Spanish al final del día can sometimes be literal like here, and it does not always carry the same idiomatic force as English.

Because the present tense of leer is:

• leo
• lees
• lee
• leemos
• leéis
• leen

So leo is the normal first-person singular form: I read.

Spanish does not insert a y there. The y appears in some other forms and tenses of leer, for example:

• leyó
• leyeron
• leyendo

But in the simple present, leo is correct.

The present tense in Spanish is commonly used for habitual actions or things someone does sometimes.

So this sentence means something like:

• Sometimes I sit in that armchair at the end of the day and read for a while to relax.

It describes a routine or repeated habit, not necessarily something happening right now.

Un rato means a while or for a bit.

It is a very common, natural expression in Spanish. It is not usually a precise amount of time; it just means for some time or for a short while.

Examples:

• Descanso un rato. = I rest for a while.
• Voy a leer un rato. = I’m going to read for a while.

So leo un rato = I read for a while.

Because in Spanish, expressions of duration often appear without a preposition.

• Leo un rato. = I read for a while.
• Esperé dos horas. = I waited for two hours.
• Dormí toda la tarde. = I slept all afternoon.

English often needs for, but Spanish frequently does not.

Para here expresses purpose: in order to or to.

So:

• leo un rato para relajarme = I read for a while to relax

This structure is very common:

• Estudio para aprender. = I study to learn.
• Salgo temprano para evitar tráfico. = I leave early to avoid traffic.

The infinitive relajarme includes me because the idea is to relax myself.

Because the intended meaning is to relax myself / to unwind. In Spanish, that idea is often expressed with the reflexive form relajarse.

So:

• relajar = to relax something / to make something relaxed
• relajarse = to relax oneself, to unwind

In this sentence:

• para relajarme = to relax, to unwind

English usually does not say to relax myself in this context, but Spanish commonly uses the reflexive form.

Yes, but the meaning would change slightly.

• para relajarme = to relax, unwind, reduce tension
• para descansar = to rest

They are close, but not identical. Reading might be done to unwind mentally, so relajarme fits very well.

Because the second verb is a new action with the same subject, I, and leer is not reflexive here.

So the structure is simply:

• me siento = I sit down
• y leo = and I read

The reflexive pronoun me belongs only to sentarse here, not to leer.

Not completely. Spanish word order is fairly flexible, though some versions sound more natural than others.

For example, these are possible:

• A veces me siento en ese sillón al final del día y leo un rato para relajarme.
• Al final del día, a veces me siento en ese sillón y leo un rato para relajarme.
• A veces, al final del día, me siento en ese sillón y leo un rato para relajarme.

The original sentence sounds natural and neutral. Moving parts around changes emphasis more than basic meaning.

Yes. Ese sillón points to a particular armchair: that armchair.

If you said el sillón, it would mean the armchair, assuming the listener already knows which one you mean.

So the difference is roughly:

• el sillón = the armchair
• ese sillón = that armchair

Using ese adds a sense of pointing out or identifying a specific one.

At first glance, yes, because me siento can also mean I feel from sentirse.

But the rest of the sentence makes the meaning clear:

• me siento en ese sillón can only mean I sit down in that armchair

If it meant I feel, you would expect something like:

• me siento cansado
• me siento bien
• me siento triste

So the following words tell you which verb is being used.

A few points stand out for Spain:

• c in veces is pronounced with the th sound of English think in most of Spain: ve-thes
• z in relajarme does not appear, so no issue there
• ll in sillón is usually pronounced like y in much of Spain, though exact pronunciation varies by region
• the stress in sillón falls on the last syllable because of the written accent: si-LLÓN

This is pronunciation, not grammar, but it is a common question for learners of Spanish from Spain.