Se tudo depender do tempo, talvez a gente viaje só amanhã.

Because this is a future condition: if everything depends on the weather in the sense of if it turns out to depend on the weather.

After se in this kind of future if clause, Portuguese normally uses the future subjunctive. For the verb depender, the ele/ela/você form of the future subjunctive is depender.

So here, depender is not the infinitive in function, even though it looks the same.

• Se tudo depender do tempo... = If everything depends / ends up depending on the weather...
• Se tudo depende do tempo... sounds more like a general present statement.

Here, se means if.

It introduces a condition:

• Se tudo depender do tempo... = If everything depends on the weather...

This is one of the most common uses of se in Portuguese.

In this sentence, tempo means weather.

Portuguese uses tempo for both time and weather, so context tells you which one is meant. Since the sentence is about whether they will travel, weather makes the most sense.

So:

• o tempo = the weather
• o tempo can also mean time, depending on context

Because talvez often triggers the subjunctive when talking about uncertainty.

Here, talvez means maybe / perhaps, so the sentence expresses possibility rather than certainty. That is why Portuguese uses:

• talvez a gente viaje

instead of:

• talvez a gente viaja
• talvez a gente viajará

In standard Portuguese, talvez + subjunctive is the expected pattern for uncertain present or future situations.

A gente is a very common Brazilian Portuguese way to say we.

Even though it means we, grammatically it takes a third-person singular verb, not a plural one.

So you get:

• a gente viaja
• a gente vai
• a gente quer

not:

• a gente viajamos
• a gente vamos

In Brazil, a gente is extremely common in everyday speech and informal writing.

Yes, absolutely.

A more formal or textbook-style version would be:

• Se tudo depender do tempo, talvez nós viajemos só amanhã.

Notice that if you change a gente to nós, the verb must also change:

• a gente viaje
• nós viajemos

Both are correct, but a gente sounds more natural in much of spoken Brazilian Portuguese.

Here, só amanhã means something like only tomorrow or not until tomorrow.

The idea is that the trip may happen tomorrow, and not earlier.

So the sentence suggests:

• maybe we will travel tomorrow only
• maybe we won’t travel before tomorrow

In natural English, this is often best understood as maybe we won’t travel until tomorrow.

Yes. That version is also very natural:

• Talvez a gente só viaje amanhã.

In fact, many learners find this version easier to understand, because só is placed directly before the verb phrase it affects.

Compare:

• talvez a gente viaje só amanhã
• talvez a gente só viaje amanhã

Both are natural and mean essentially the same thing here.

No.

• tudo = everything
• todo = all / every / whole

In this sentence, tudo is correct because it stands alone and means everything:

• Se tudo depender do tempo... = If everything depends on the weather...

You would use todo with a noun, for example:

• todo o plano = the whole plan
• todo dia = every day

It is not exactly wrong, but it means something a little different.

• Se tudo depender do tempo... = a future condition
• Se tudo depende do tempo... = more like a present or general situation

Since the sentence is talking about a possible future trip, depender in the future subjunctive is the natural choice.

So for this context, Se tudo depender do tempo... is the best form.