Если мы застрянем в пробке, мы опоздаем на поезд.

Because Russian usually uses the future in both parts of a real future condition.

So Russian says, literally:

• Если мы застрянем в пробке, мы опоздаем на поезд.
• If we get stuck in traffic, we will be late for the train.

In English, after if, we normally use the present: If we get stuck...
In Russian, that same idea is normally expressed with the future:

• если + future
• future

This is completely normal Russian grammar.

They are perfective because the sentence talks about single complete events with a result:

• застрянем = we will get stuck
• опоздаем = we will end up being late / miss it

Perfective is very common when you mean a one-time future event.

Compare:

• Если мы застрянем в пробке, мы опоздаем на поезд.
  = one specific situation in the future

But if you were talking about something habitual, you would more likely use imperfective:

• Если мы застреваем в пробке, мы опаздываем на поезд.
  = whenever we get stuck in traffic, we are late for the train

So in your sentence, perfective fits because it is about one possible future occurrence.

The infinitive is застрять.

Застрянем is the 1st person plural future form of this perfective verb:

• я застряну
• мы застрянем

It may look unusual because the stem changes a little compared with the infinitive. This is just part of the verb’s conjugation pattern.

A very useful pair to know is:

• застрять — perfective
• застревать — imperfective

So:

• Мы застрянем = we will get stuck
• Мы застреваем = we get stuck / are getting stuck

It is prepositional case after в, because the phrase means being in a traffic jam.

• пробка = traffic jam
• в пробке = in a traffic jam

The full expression застрять в пробке is very common and idiomatic. You should learn it as a set phrase:

• застрять в пробке = to get stuck in traffic

Also, remember that пробка can mean other things too, such as cork or bottle cap, but in this context it means traffic jam.

After опоздать, Russian often uses на + accusative for transport, events, lessons, flights, trains, and so on:

• опоздать на поезд = be late for the train / miss the train
• опоздать на автобус = be late for the bus
• опоздать на самолёт = be late for the plane
• опоздать на урок = be late for class

Here поезд is in the accusative, but because it is an inanimate masculine noun, the accusative looks the same as the nominative:

• nominative: поезд
• accusative: поезд

So the form does not change, even though the case does.

It can cover both ideas, depending on context.

Very often, опоздать на поезд implies that you were too late and therefore missed it. But literally it is to be late for the train.

So in many situations, English might translate it as either:

• we’ll be late for the train
• we’ll miss the train

Both can fit, depending on the situation.

Yes, it could be omitted.

Russian often drops subject pronouns when the verb ending already makes the subject clear. So this is also natural:

• Если застрянем в пробке, опоздаем на поезд.

That means the same thing.

The repeated мы is not wrong at all. It can make the sentence a bit clearer, a bit more balanced, or slightly more emphatic:

• Если мы застрянем в пробке, мы опоздаем на поезд.

So both versions are fine.

Because Если мы застрянем в пробке is a subordinate clause introduced by если.

Russian normally puts a comma between the if-clause and the main clause:

• Если мы застрянем в пробке, мы опоздаем на поезд.

If the order is reversed, the comma still appears:

• Мы опоздаем на поезд, если застрянем в пробке.

So the comma is required here.

Not in this sentence, if you mean a real future possibility.

• Если = if
• Если бы = if ... were / if ... had / if only ... ; it introduces a more hypothetical or unreal situation

Your sentence is about a real possible future event, so plain если is correct:

• Если мы застрянем в пробке, мы опоздаем на поезд.

If you wanted a more hypothetical or contrary-to-fact meaning, you would use если бы and past-tense forms with бы:

• Если бы мы застряли в пробке, мы бы опоздали на поезд.
• If we got stuck in traffic, we would miss the train.
• or, depending on context, If we had gotten stuck in traffic, we would have missed the train.

So если and если бы are not interchangeable.

Yes. Russian word order is fairly flexible.

A very common alternative is:

• Мы опоздаем на поезд, если застрянем в пробке.

This means the same thing.

You can change word order for focus or style, but the original version is very standard and natural because it presents the condition first, then the result.

So both are good:

• Если мы застрянем в пробке, мы опоздаем на поезд.
• Мы опоздаем на поезд, если застрянем в пробке.

It is pronounced approximately za-STRYA-nyem.

What makes it tricky for English speakers:

• the consonant cluster стр
• the я sound after р
• the softening before -нем

A rough breakdown:

• за — like za
• стря — stressed part, roughly STRYA
• нем — roughly nyem

So: застрянем

It helps to practice it slowly:

• за-стря-нем
• застрянем

This is one of those forms that becomes much easier once you have heard and repeated it several times.