В переполненном вагоне я держался за поручень и ждал, пока освободится сиденье.

Because в here means in, so вагон takes the prepositional case:

• вагон → в вагоне

The adjective has to agree with the noun in case, number, and gender:

• переполненный вагон → в переполненном вагоне

So the whole phrase means in the overcrowded/crowded carriage.

Вагон usually means a railway car / train car / carriage. In this sentence it refers to the inside of a train or metro car, not the whole train.

Depending on context, English translations could be:

• car
• carriage
• train car
• railway carriage

Because держаться за что-то is the normal Russian expression for to hold on to something.

Compare:

• держать что-то = to hold something
• держаться за что-то = to hold on to something

So:

• я держал поручень = I was holding the handrail
• я держался за поручень = I was holding on to the handrail

In a crowded train, держаться за поручень is the more natural choice.

Also, -ся here is not really reflexive in the English sense of myself. It is just part of the verb form.

The verb phrase is держаться за + accusative.

So:

• за поручень
• за ручку
• за перила

That is simply the pattern Russian uses with this meaning: to hold on to.

A learner might expect another case because за can take different cases in different meanings, but with держаться за you normally use the accusative.

Поручень means a handrail, rail, or grab bar—something you hold onto for support, especially in transport.

In a bus, train, or metro, it is the bar or rail passengers hold when standing.

Because the whole sentence describes what the speaker was doing at that time in the past.

• держался = was holding on
• ждал = was waiting

Both are past tense, masculine singular forms.

Russian often uses the past tense imperfective here to describe an ongoing situation:

• he was standing there
• holding on
• waiting

If the speaker were female, it would be:

• держалась
• ждала

It can mean while or until, depending on context.

Here it means until:

• ждал, пока освободится сиденье = waited until a seat became free / until a seat was free

Why until here? Because the second action marks the point the waiting ends: the seat becomes available.

Compare:

• Я читал, пока он готовил ужин. = I was reading while he was cooking dinner.
• Я ждал, пока он придёт. = I waited until he came.

So пока is flexible, and the verb forms help show which meaning is intended.

This is a very common Russian pattern.

After пока meaning until, Russian often uses a future perfective verb to refer to the event that will happen after the waiting starts.

So:

• ждал, пока освободится сиденье

Literally, from the point of view of the waiting, the seat will become free.

In natural English, we usually translate it as:

• waited until a seat became free or
• waited until a seat was available

So even though the main sentence is in the past, Russian uses future perfective in the subordinate clause because it describes the event that had not happened yet at that point.

Because the sentence is about a single completed change of state: the moment when the seat becomes free.

• освобождаться = to be getting freed / to become free repeatedly or as a process
• освободиться = to become free, to be freed completely

Here the speaker is waiting for one clear result:

• a seat becomes available

So perfective is the natural choice.

Because сиденье is the subject of the clause пока освободится сиденье.

It is the seat that becomes free.

So:

• сиденье освободится = the seat will become free

That is why it is nominative, not accusative.

Yes, but the meaning shifts slightly.

• сиденье = the seat itself
• место = a place/spot/seat space

In public transport, both can work:

• пока освободится сиденье
• пока освободится место

But место is often more idiomatic when you mean an available seat/spot in general, while сиденье points more directly to the physical seat.

Yes, Russian word order is flexible.

This version starts with В переполненном вагоне to set the scene first:

• In the crowded carriage, ...

That sounds natural because it gives the location/background before the action.

Other possible orders are grammatically fine, for example:

• Я держался за поручень и ждал, пока освободится сиденье, в переполненном вагоне.
• Я в переполненном вагоне держался за поручень...

But the original word order is smooth and natural because it introduces the setting first.

Russian expresses that idea through the combination of the verb and preposition:

• держаться за = to hold on to

So English uses a separate phrase like hold onto or hold on to, while Russian uses:

• держаться за

You do not need an extra word beyond за.

Yes.

• полный = full
• переполненный = overfilled, overcrowded, packed

So переполненный вагон suggests a carriage that is not just full, but very crowded.

That fits well with the rest of the sentence: the speaker is standing, holding a rail, and waiting for a seat.

Yes, that is a very natural way to understand it.

Possible English renderings include:

• I waited until a seat became free
• I waited for a seat to free up
• I waited until a seat opened up
• I waited until a seat became available

All of these match the Russian well.