Проверь, закрыта ли застёжка на рюкзаке, а то вещи могут выпасть.

Проверь is the imperative form of проверить, which means to check.

So Проверь means Check! when speaking to one person informally.

If you wanted to say it politely or to more than one person, you would say Проверьте.

• проверить = to check, to verify
• Проверь = check!

The particle ли is used to introduce an indirect yes/no question.

So закрыта ли застёжка means something like:

• whether the fastener is closed
• if the clasp is closed

This is different from a direct question:

• Застёжка закрыта? = Is the fastener closed?

After verbs like проверь (check), Russian often uses this X ли... structure:

• Проверь, дома ли он. = Check whether he is at home.
• Спроси, готова ли еда. = Ask whether the food is ready.

In this sentence, ли goes after закрыта because that is the element being questioned.

Закрыта is the short form of закрытая and means closed.

Here it agrees with застёжка, which is:

• feminine
• singular

So:

• застёжка закрыта = the fastener is closed

Compare the short forms:

• закрыт — masculine
• закрыта — feminine
• закрыто — neuter
• закрыты — plural

Russian often uses these short-form adjectives or participles in statements about a state or condition:

• Дверь закрыта. = The door is closed.
• Окно открыто. = The window is open.

Застёжка is a general word for a fastener, clasp, closure, or something used to fasten.

In the context of a backpack, it could refer to:

• a zipper
• a buckle
• a clasp
• some other fastening mechanism

It is broader than one exact English word. Russian often uses general practical nouns like this where English might choose something more specific from context.

На рюкзаке means on the backpack.

That makes sense because the fastener is considered to be attached to the outside/surface of the backpack, not inside it.

So:

• застёжка на рюкзаке = the fastener on the backpack

Compare:

• в рюкзаке = in the backpack
• на рюкзаке = on the backpack

Russian uses на with the prepositional case here because it is talking about location.

Рюкзаке is the prepositional singular form of рюкзак.

The preposition на often requires the prepositional case when it means on / at in a location sense.

So:

• рюкзак = backpack
• на рюкзаке = on the backpack

This is a standard pattern:

• на столе = on the table
• на двери = on the door
• на рюкзаке = on the backpack

А то means something like:

• or else
• otherwise
• because if not

In this sentence:

• Проверь, закрыта ли застёжка на рюкзаке, а то вещи могут выпасть.
• Check whether the fastener on the backpack is closed, otherwise the things might fall out.

It introduces a possible negative consequence.

This is a very common conversational structure in Russian.

Examples:

• Надень куртку, а то замёрзнешь. = Put on a jacket, or you’ll get cold.
• Поторопись, а то опоздаем. = Hurry up, or we’ll be late.

Могут means can / may / might.

So вещи могут выпасть means:

• the things may fall out
• your stuff might fall out

It expresses possibility, not certainty.

Word by word:

• вещи = things, belongings, stuff
• могут = can / may / might
• выпасть = to fall out

So the speaker is warning about a possible result if the fastener is not closed.

Выпасть is a perfective verb meaning to fall out as a complete event.

Here that fits well because the sentence is about a possible result:

• the things might fall out

Russian often uses the perfective infinitive after мочь when talking about a possible single completed event:

• могу забыть = I might forget
• может сломаться = it may break
• могут выпасть = they might fall out

Compare:

• выпасть = to fall out completely / to come out
• выпадать = to be falling out / to fall out repeatedly or habitually

Also, падать just means to fall, but выпасть specifically adds the idea of falling out of something.

There is a comma because Проверь is followed by a subordinate clause:

• Проверь, закрыта ли застёжка на рюкзаке

Literally: Check, whether the fastener on the backpack is closed

Russian normally separates this kind of subordinate clause with a comma.

There is also a comma before а то, because а то introduces another clause:

• ..., а то вещи могут выпасть

So the commas are doing normal clause-separating work.

Yes. Russian has several natural alternatives, depending on tone.

For example:

• Проверь, застёгнут ли рюкзак... = Check whether the backpack is fastened...
• Проверь, закрыт ли рюкзак... = Check whether the backpack is closed...
• Проверь, закрыта ли молния на рюкзаке... = Check whether the zipper on the backpack is closed...

The original sentence focuses specifically on the fastener itself, not just the backpack as a whole.

In context, вещи often means things, stuff, or belongings.

With a backpack, English would often naturally say:

• your things
• your stuff
• your belongings

So although the literal meaning is things, the practical meaning is more like the items in the backpack.

The sentence breaks down like this:

• Проверь = Check
• закрыта ли застёжка на рюкзаке = whether the fastener on the backpack is closed
• а то = otherwise / or else
• вещи могут выпасть = things might fall out

So the structure is:

imperative + indirect yes/no question + warning about a consequence

That is a very common Russian pattern:

• Сделай X, а то случится Y.
• Do X, or else Y may happen.

It is informal, because it uses Проверь, the singular informal imperative.

To make it polite or formal, you would say:

• Проверьте, закрыта ли застёжка на рюкзаке, а то вещи могут выпасть.

Everything else in the sentence can stay the same.

So the only thing that clearly marks informality here is Проверь.