Agosto é o mês em que eu volto ao trabalho com mais calma.

Yes. In Portuguese, months are normally written in lowercase: agosto, setembro, janeiro.

Here, Agosto is capitalized only because it is the first word of the sentence. If it appeared in the middle, it would usually be agosto:

Eu volto ao trabalho em agosto.

Because Portuguese normally uses the definite article here: o mês = the month.

So:

Agosto é o mês... = August is the month...

Saying Agosto é mês... would sound incomplete or unnatural in this context. Portuguese often uses articles in places where English may or may not.

Em que here means in which or, more naturally in English, when.

So:

o mês em que eu volto ao trabalho
= the month in which I return to work
= the month when I go back to work

It is a very common structure in Portuguese after nouns of time or place:

• o dia em que = the day when
• a cidade em que = the city where / the city in which
• o momento em que = the moment when

Sometimes native speakers do say o mês que, especially in informal speech, but o mês em que is more standard and clearer.

Quando is also possible in some contexts:

• o mês em que eu volto ao trabalho
• o mês quando eu volto ao trabalho (less standard for many speakers)

For a learner, em que is the safest and most widely acceptable choice here.

Yes, it could.

Portuguese often drops the subject pronoun because the verb ending already shows who the subject is:

• eu volto = I return / I go back
• volto already tells us it is I

So both are possible:

• Agosto é o mês em que eu volto ao trabalho.
• Agosto é o mês em que volto ao trabalho.

Including eu can add a little emphasis, clarity, or natural rhythm, but it is not required.

Good question. Portuguese often uses the present tense to talk about:

1. habitual actions
2. scheduled or expected future actions
3. general personal patterns

So volto here can mean something like:

• I go back
• I return
• I’m going back

Depending on context, it may refer to a regular yearly pattern: every August, I go back to work more calmly.

If you said vou voltar, that would sound more like a specific future event. The present tense here sounds very natural for a recurring situation.

Volto ao trabalho means I return to work or I go back to work.

It comes from:

• voltar = to return / to go back
• a o = ao = to the

So:

• volto ao trabalho = I return to work
• literally: I return to the work

In natural English, go back to work is often the best translation.

Because ao is the contraction of a + o:

• a = to
• o trabalho = the work
• a + o = ao

So:

voltar a algum lugar = to return to some place/state

Here, trabalho is treated as the destination/state you return to:

• voltar ao trabalho = to go back to work

Compare:

• no trabalho = at work / in the workplace
• ao trabalho = to work / back to work

So volto no trabalho would not mean the same thing.

They are similar, but not identical.

• volto ao trabalho = I go back to work
• volto a trabalhar = I start working again / I go back to working

The first one focuses more on returning to work as a routine, job, or workplace.
The second focuses more on the action of working again.

In many situations, both are possible, but voltar ao trabalho is very common when talking about returning after vacation or time off.

Com mais calma literally means with more calmness, but in natural English it usually means:

• more calmly
• more relaxed
• at a calmer pace
• without so much rush

So the sentence suggests that when the speaker goes back to work in August, they do it in a more relaxed, less hectic way.

It is a very common expression in Portuguese:

• Vamos conversar com mais calma. = Let’s talk more calmly / when things are calmer.
• Depois eu vejo isso com mais calma. = I’ll look at that more carefully later.

Because calma here is a noun, not an adjective.

• calmo / calma as an adjective = calm
• a calma as a noun = calmness

In com mais calma, the idea is:

• with more calmness
• with greater calm

That is why the feminine form calma appears: it matches the noun a calma.

By itself, it usually sounds like a general or recurring personal fact:

August is the month when I go back to work more calmly.

The use of é and volto in the present tense often gives it a habitual feeling.

But context can make it refer to a particular year too. For example, if someone is talking about their current schedule, it could also mean something like:

This year, August is the month when I’m going back to work more calmly.

Without more context, the most natural reading is a general pattern.

Yes. A simpler version would be:

Em agosto eu volto ao trabalho com mais calma.

This means roughly the same thing:
In August I go back to work more calmly.

The original sentence, Agosto é o mês em que..., puts more emphasis on August as the particular month. It sounds a bit more descriptive or reflective, while Em agosto... is more direct.