Learn Hardcore Portuguese (Portugal): Para nós chegarmos a horas, é preciso sairmos de casa mais cedo. - For us to arrive on time, we need to leave the house earlier.

Para chegarmos a horas…
→ still clearly “we” from the context (most natural here).
Para chegar a horas…
→ more generic: “to arrive on time (in general / one should…)”, which could be “I/you/we/people” depending on context.

In your sentence, “para nós chegarmos” makes the subject very explicit: it’s specifically us, not people in general.

What exactly is the personal / inflected infinitive and why is it used here?

The infinitivo pessoal (personal infinitive) is an infinitive that changes form according to the subject, like:

eu chegar
tu chegares
ele/ela/você chegar
nós chegarmos
vós chegardes
eles/elas/vocês chegarem

You use it especially:

After certain prepositions (para, por, sem, ao, antes de, depois de, etc.)
When you want to show clearly who does the action.

In your sentence:

Para nós chegarmos a horas…
→ “For us to arrive on time…”

The personal infinitive is ideal because:

It comes after a preposition (para), and
The subject is known (nós), so the verb agrees with it (chegarmos).

Could I say “para que nós cheguemos a horas” instead of “para nós chegarmos a horas”?

Yes. That is grammatically correct and perfectly understandable:

Para que nós cheguemos a horas, é preciso sairmos de casa mais cedo.

Difference in feel:

para nós chegarmos (preposition + personal infinitive)
→ very common, quite natural, slightly more neutral/colloquial.
para que nós cheguemos (conjunction + present subjunctive)
→ feels a bit more formal or literary in modern European Portuguese.

In everyday EP speech and writing, you’ll usually hear “para (nós) chegarmos…” more than “para que nós cheguemos…”.

Can I drop the “nós” and just say “Para chegarmos a horas…”?

Yes, and that’s actually the most natural wording in many contexts:

Para chegarmos a horas, é preciso sairmos de casa mais cedo.

Why is “nós” often dropped?

The form “chegarmos” already tells you it’s “we”.
Portuguese tends to omit subject pronouns when the verb ending already makes the subject clear.

So you can say:

Para nós chegarmos a horas… (more emphasis on us specifically)
Para chegarmos a horas… (normal, smooth, slightly more natural style)

Why do we also have “sairmos” and not just “sair” after “é preciso”?

Again, this is the personal infinitive agreeing with the implied subject “nós”.

É preciso sairmos de casa mais cedo.
Literally: It is necessary for us to leave home earlier.

Compare:

É preciso sair de casa mais cedo.
→ More generic: “It’s necessary to leave home earlier (one/you/we have to leave…)”
É preciso sairmos de casa mais cedo.
→ The subject is specifically implied we.

Both are correct; in your full sentence, sairmos matches the “nós” from the first clause, keeping everything nicely aligned with the same subject.

Could I use the subjunctive instead of the infinitive after “é preciso”, like “é preciso que saiamos de casa”?

Yes, that’s also correct:

É preciso que saiamos de casa mais cedo.

Differences:

É preciso sairmos de casa…
→ Impersonal expression + personal infinitive; very common and natural.
É preciso que saiamos de casa…
→ Impersonal expression + subjunctive clause; often a bit more formal or emphatic.

Meaning-wise, both are “We need to leave home earlier” / “It is necessary that we leave home earlier.”
In everyday speech, “é preciso sairmos de casa…” is probably more frequent.

What does “a horas” mean exactly, and why is it “a horas” and not “às horas”?

In European Portuguese, “chegar a horas” is a fixed expression meaning:

“to be on time / to arrive on time”

Some key points:

“chegar a horas” → idiomatic phrase: on time.
“a” here is just the simple preposition a.
It is not the contraction “às” (a + as), which you use with a specific time:
- chegar às três horas = arrive at three o’clock.

So:

chegar a horas = arrive on time (no late)
chegar às horas X = arrive at the hours X (e.g., às oito horas)

You could also hear:

chegar a tempo = also “to arrive in time / on time”, often with a nuance of “before it’s too late / before something starts”.

Is there any difference between “chegar a horas” and “chegar a tempo”?

Both can be translated as “to arrive on time”, but there’s a nuance:

chegar a horas
→ more about punctuality, not being late for something scheduled.
chegar a tempo
→ more about arriving in time to do something (before a deadline, before it starts/ends).

Examples:

Cheguei a horas ao trabalho.
→ I wasn’t late for work (I arrived at the correct, expected time).
Cheguei a tempo de apanhar o comboio.
→ I got there in time to catch the train (before it left).

In your sentence, “chegarmos a horas” fits well because it’s simply “so that we’re not late / so that we arrive punctually.”

Why is it “de casa” and not “da casa”?

In Portuguese, with the verb “sair” in the sense of “leave home / leave the house where you live”, the standard idiom is:

sair de casa = to leave home.

We normally don’t say “sair da casa” in this generic sense of “leaving home you live in”.
“da casa” (de + a) would sound like you’re talking about a particular house as an object or building:

Sair da casa azul. = leave the blue house (a specific house, not “home” in general).

So:

sair de casa = leave home (generic, where you live).
sair da casa = leave the (particular) house.

Can I change the order of the clauses, like: “É preciso sairmos de casa mais cedo para nós chegarmos a horas”?

Yes, that word order is also correct:

É preciso sairmos de casa mais cedo para (nós) chegarmos a horas.

Both:

Para (nós) chegarmos a horas, é preciso sairmos de casa mais cedo.
É preciso sairmos de casa mais cedo para (nós) chegarmos a horas.

are natural. The difference is just what you emphasize first:

Starting with “Para chegarmos a horas…” emphasizes the goal.
Starting with “É preciso sairmos de casa…” emphasizes the condition / requirement.

Why is the comma used after “a horas”?

The first part:

Para nós chegarmos a horas

is an adverbial clause of purpose/condition placed at the beginning of the sentence. In Portuguese, it’s standard to separate such a fronted subordinate clause with a comma:

Para nós chegarmos a horas, é preciso sairmos de casa mais cedo.

If you reverse the order, you often don’t need a comma:

É preciso sairmos de casa mais cedo para nós chegarmos a horas.

So the comma is there mainly for clarity and standard punctuation, not for any special grammatical meaning.