Em setembro e outubro, eu passo a usar o autocarro para chegar a horas.

Yes. In this sentence, em means in or during with months.

So:

• em setembro = in September
• em outubro = in October

Portuguese normally uses em with months, seasons, and many time expressions.

Also, it is very normal to say em setembro e outubro with just one em covering both months. You could also say em setembro e em outubro, but that is less economical and usually only used for extra emphasis or contrast.

In Portuguese, the names of months and days of the week are normally written in lowercase, not capitals.

So:

• setembro
• outubro
• segunda-feira

This is different from English, where September and October must be capitalized.

Not usually. Portuguese verb endings often already show who the subject is.

• passo already means I pass / I start / I begin
• so eu passo and passo can both mean I ...

That means you could say:

• Em setembro e outubro, passo a usar o autocarro para chegar a horas.

and it would still be clear.

Using eu is not wrong at all. It can sound slightly more explicit, emphatic, or just stylistically natural in context.

Passar a + infinitive is a very common Portuguese structure meaning:

• to start doing
• to begin to
• to come to
• sometimes to switch to

So:

• passo a usar o autocarro = I start using the bus / I switch to taking the bus

This is not the same as just I use the bus. It suggests a change: before, the speaker did something else, and now begins this new habit.

Because uso o autocarro and passo a usar o autocarro do not mean exactly the same thing.

• uso o autocarro = I use the bus
• passo a usar o autocarro = I start using the bus / I switch to using the bus

The version with passo a adds the idea of a new routine or change of behavior.

So if the speaker is saying that in September and October they begin taking the bus, passo a usar is the better choice.

Portuguese very often uses the definite article with nouns where English would not.

So usar o autocarro is the natural way to say use the bus.

Here, o does not necessarily mean one specific bus vehicle. It often works more generally, referring to the bus as a means of transport.

This kind of article use is very common in Portuguese.

Yes. Autocarro is the standard European Portuguese word for bus.

In Brazilian Portuguese, the usual word is:

• ônibus

So:

• European Portuguese: autocarro
• Brazilian Portuguese: ônibus

If you are learning Portuguese from Portugal, autocarro is the right word to learn.

Para + infinitive often expresses purpose — in other words, in order to.

So:

• para chegar a horas = to arrive on time / in order to arrive on time

It explains why the speaker starts using the bus.

Structure:

• [main action] + para + infinitive
• passo a usar o autocarro para chegar a horas
• I start taking the bus to get there on time

In European Portuguese, chegar a horas is a very common expression meaning:

• to arrive on time
• to not be late

So:

• para chegar a horas = so as to arrive on time

This is slightly idiomatic, so it is best learned as a chunk:

• chegar a horas

It is not exactly the same as every other time expression:

• a horas = on time, punctually, not late
• a tempo = in time, before it is too late
• na hora = at the appointed time / at that exact time, depending on context

In this sentence, a horas is very natural.

Portuguese, like English, often uses the present tense for:

• habits
• routines
• scheduled or regular actions
• statements about what someone does during a certain period

So passo here can mean something like:

• In September and October, I start taking the bus...
• or During September and October, I switch to taking the bus...

It presents the action as part of the speaker’s routine or pattern for that period.

If you wanted a clearly future meaning in another context, Portuguese might also use the present tense anyway, especially when the time phrase already makes the timing clear.

Yes. Portuguese word order is fairly flexible.

For example:

• Em setembro e outubro, eu passo a usar o autocarro para chegar a horas.
• Eu passo a usar o autocarro em setembro e outubro para chegar a horas.

Both are possible.

Putting Em setembro e outubro at the beginning is very natural because it sets the time frame first. That is often the most natural order when the speaker wants to establish the context before the main information.

Yes, depending on context, em setembro e outubro can naturally suggest during those months or throughout that period.

It does not usually mean one single moment in September and one single moment in October. It more often describes a routine or state that applies during that time span.

Because the verb phrase passo a usar suggests a habit or change in routine, the overall meaning is very naturally understood as something that happens over that period.

It is especially common and very natural in European Portuguese.

A learner of Portuguese from Portugal should definitely know it. It is one of those everyday expressions that native speakers use a lot.

Even if other varieties may understand it, it is particularly useful to remember as an EP phrase:

• Chega a horas! = Arrive on time!
• Espero chegar a horas. = I hope to arrive on time.

So in your sentence, it sounds very idiomatic and native-like.