Da quando è iniziato luglio, Marta beve più acqua e torna tardi solo ad agosto.

Da quando means since or more literally from the time when.

In this sentence, Da quando è iniziato luglio means since July started / since the beginning of July.

A few useful comparisons:

• quando = when
• da = from / since
• da quando = since when / from the time when

So da quando introduces the starting point of a situation that continues afterward.

Because iniziare can be used intransitively here, meaning to begin.

In è iniziato luglio, the subject is luglio. Literally: July began.

Many intransitive verbs of change or movement use essere in the passato prossimo, so:

• luglio è iniziato = July started
• not luglio ha iniziato, which would normally need an object and would mean something like July started something, which does not make sense here.

So:

• È iniziato luglio = July has started / July started

The subject comes after the verb, which is very normal in Italian.

This is a very common Italian pattern.

Italian often uses:

• a past tense for the starting point
• the present tense for actions or situations that began in the past and are still true now

So:

• Da quando è iniziato luglio, Marta beve più acqua
  literally: Since July started, Marta drinks more water
• in natural English, this is often Since July started, Marta has been drinking more water

Italian does not always use a present perfect where English does. The present tense often covers the idea of has been doing if the action still continues.

The same applies to torna if the speaker is describing her current habit or schedule.

In Italian, the names of months usually do not take an article.

So you normally say:

• luglio
• agosto
• a luglio
• ad agosto

not:

• il luglio
• l’agosto

unless you are using the month name in a special context, for example talking about a specific July already mentioned, or using it as a noun with extra description.

In this sentence, plain month names are exactly what you want.

Because a often becomes ad before a vowel for smoother pronunciation.

So:

• a + agosto → ad agosto

This is very common and standard.

You will also see:

• ad aprile
• ad ottobre

In modern Italian, the extra d is not always required before vowels in every expression, but with months like agosto, ad agosto is very normal and natural.

Più here means more.

So:

• beve più acqua = she drinks more water

This is a comparative idea: more than before, or more than some other amount understood from context.

It does not mean the most. That would be something different, such as:

• beve più acqua di prima = she drinks more water than before
• beve più acqua di Luca = she drinks more water than Luca

In your sentence, the comparison is implied rather than stated.

Here solo most naturally modifies ad agosto.

So the meaning is:

• she comes back late only in August

In other words, August is the only time when she comes back late.

This is different from other possible placements:

• solo torna tardi would sound unusual and would suggest she only does one thing: come back late
• torna solo tardi would mean she comes back only late, focusing more directly on late

In your sentence, solo ad agosto is best understood as only in August.

It is grammatically correct, but a learner might notice that it feels a bit unusual in meaning.

Why? Because:

• Da quando è iniziato luglio sets a starting point: since July began
• solo ad agosto points to only in August

So the sentence mixes a change starting in July with a restriction to August. That can work, but it depends on context.

For example, maybe the speaker means:

• since July began, Marta has been drinking more water
• and her schedule is such that she comes home late only in August

So the grammar is fine, but the two time ideas are a little separate.

A more straightforward sentence might be something like:

• Da quando è iniziato luglio, Marta beve più acqua. Torna tardi solo ad agosto.

That makes the two statements easier to process.

Yes, very often da luglio would be a simpler and more natural alternative.

Compare:

• Da quando è iniziato luglio = since July started
• Da luglio = since July

Both can work, but they are slightly different:

• da luglio is shorter and more general
• da quando è iniziato luglio emphasizes the exact starting moment: the point when July began

So if you want a cleaner everyday phrasing, you could say:

• Da luglio, Marta beve più acqua...

But the original sentence is still valid.

Yes, it could be omitted if the subject is already clear.

Italian often drops subject pronouns and even proper nouns when context makes the subject obvious.

So both are possible:

• Da quando è iniziato luglio, Marta beve più acqua e torna tardi solo ad agosto.
• Da quando è iniziato luglio, beve più acqua e torna tardi solo ad agosto.

The first version is clearer if you are introducing Marta or contrasting her with someone else. The second is more natural if everyone already knows you are talking about Marta.

Both are possible translations.

Tornare often means:

• to return
• to come back
• sometimes to go back, depending on context

In everyday English, comes back late or gets back late usually sounds more natural than returns late.

So:

• torna tardi = she comes back late / she gets back late

If the context is about going home, that is probably the most natural interpretation.

Yes, Italian word order is fairly flexible, but changing it can change the emphasis.

Original:

• Marta beve più acqua e torna tardi solo ad agosto.

This most naturally means:

• Marta drinks more water and comes back late only in August.

You could also say:

• Solo ad agosto Marta torna tardi.

This puts stronger emphasis on only in August.

Or:

• Marta solo ad agosto torna tardi.

This is also possible, though a bit more marked stylistically.

So the original word order is perfectly fine, but moving solo ad agosto changes what gets highlighted.