A medida que avanza el día, me siento más tranquilo.

Yes. A medida que is a fixed expression meaning as, as/while, or more precisely in proportion as / gradually as.

In this sentence, it shows that one thing changes progressively alongside another:

• A medida que avanza el día... = As the day goes on...
• A medida que pasa el tiempo... = As time passes...

It is very common in Spanish for describing gradual change.

Because the subject is el día, which is third person singular.

So the verb avanzar is conjugated as:

• yo avanzo
• tú avanzas
• él/ella/usted avanza

Here, the day is what is advancing or moving forward, so Spanish uses avanza.

It is a normal Spanish way of speaking. Spanish often uses verbs like avanzar, pasar, or transcurrir with time expressions.

So these are all natural:

• avanza el día
• pasa el día
• transcurre el día

In English, we usually say the day goes on or the day passes, but Spanish can literally say the day advances.

Because Spanish usually uses the definite article with nouns more often than English does.

Here, el día means the day, but in context it is not necessarily a specific named day. It refers to the day as it unfolds.

Spanish commonly uses the article in this kind of general or natural expression:

• Me gusta el café.
• La vida es corta.
• Avanza el día.

English often drops the in places where Spanish keeps it.

Because sentirse means to feel, while estar means to be.

• Me siento más tranquilo = I feel calmer
• Estoy más tranquilo = I am calmer

Both can be correct, but they are not identical.

Me siento focuses more on your internal feeling or emotional state.

Estoy states the condition more directly.

So in this sentence, me siento is very natural because the speaker is describing how they feel as the day progresses.

The verb here is actually sentirse. It is a stem-changing verb: e → ie in many present-tense forms.

So:

• yo me siento
• tú te sientes
• él/ella se siente
• nosotros nos sentimos
• vosotros os sentís
• ellos se sienten

That is why it is siento, not sento.

Because sentirse is a pronominal verb, meaning it is normally used with reflexive-type pronouns:

• me siento
• te sientes
• se siente
• nos sentimos
• os sentís
• se sienten

In this structure, sentirse + adjective means to feel + adjective:

• Me siento cansado = I feel tired
• Se siente mejor = He/She feels better

So the me is required here.

Because the adjective must agree with the person who is speaking.

In me siento más tranquilo, the speaker is grammatically masculine singular.

So:

• a man would usually say me siento más tranquilo
• a woman would usually say me siento más tranquila

If the subject were plural:

• Nos sentimos más tranquilos = masculine or mixed group
• Nos sentimos más tranquilas = all-female group

The adjective agrees with the person or people feeling that way, not with el día.

It means calmer than before. Spanish often leaves the comparison implicit when it is obvious from context.

So más tranquilo does not need an explicit than phrase.

It is understood as:

• more calm than earlier
• calmer as the day goes on

If you wanted to make the comparison explicit, you could say something like:

• me siento más tranquilo que esta mañana

But in the original sentence, that comparison is simply understood.

Yes, and it is very natural.

Compare:

• A medida que avanza el día, me siento más tranquilo.
• A medida que avanza el día, me siento cada vez más tranquilo.

Cada vez más tranquilo adds the idea of increasingly calmer or more and more calm.

The original sentence is already correct and natural; cada vez just makes the gradual change even more explicit.

Because the sentence describes a present or habitual process.

• avanza = the day advances / goes on
• me siento = I feel

This can mean:

1. something happening right now, or
2. a general pattern that tends to happen

Spanish uses the present tense very naturally for both.

If you wanted to talk about the past, you would change the tenses:

• A medida que avanzaba el día, me sentía más tranquilo.

It is standard and natural here because A medida que avanza el día is an introductory subordinate clause.

So this punctuation is normal:

• A medida que avanza el día, me siento más tranquilo.

If you reverse the order, the comma is usually not needed:

• Me siento más tranquilo a medida que avanza el día.

Both word orders are correct.

Yes. Some common alternatives are:

• conforme
• según
• sometimes a medida que is still the most neutral and clear choice

For example:

• Conforme avanza el día, me siento más tranquilo.
• Según avanza el día, me siento más tranquilo.

In Spain, según in this kind of sentence can sound quite natural. But a medida que is very standard and widely understood everywhere.

Because it is pronounced in two syllables: dí-a.

The accent mark shows that the i is stressed and prevents it from merging into a diphthong. So it is not pronounced like one smooth syllable.

This is why you write:

• día
• frío
• tío

The written accent helps show the correct pronunciation.