Om du hittar lite växel i ryggsäcken, kan du köpa en flaska vatten åt mig?

Because Swedish normally uses verb-second (V2) word order in main clauses.

The first part, Om du hittar lite växel i ryggsäcken, is a subordinate clause. When that clause comes first, the finite verb of the main clause must come immediately after it:

• Om du hittar lite växel i ryggsäcken, kan du köpa ...

If you started with the main clause instead, you would get normal subject-verb order:

• Du kan köpa en flaska vatten åt mig om du hittar lite växel i ryggsäcken.

So the inversion in kan du is caused by the om-clause being placed first.

Here om means if.

• Om du hittar ... = If you find ...

In this sentence it introduces a condition: buying the water depends on whether the person finds some change in the backpack.

Swedish often uses the present tense for future meaning when the context already makes the future clear.

So:

• Om du hittar lite växel ... literally looks like If you find some change ...
• but in context it means If you happen to find some change later / when you look

English does something similar in if-clauses:

• If you find some change, can you buy...

  You do not normally say if you will find in English, and Swedish works similarly here.

Växel here means change, as in small coins or small money.

• lite växel = some change / a little change / some spare change

This is a very natural phrase in Swedish. Växel can also mean other things in other contexts, such as gear, switch, or exchange, but in this sentence it clearly means money in coins or small denominations.

Because växel meaning change is usually treated like an uncountable mass noun here, similar to English change.

So Swedish commonly says:

• lite växel = some change

Using några would normally go with countable nouns. Since the idea is not a few separate changes but rather some amount of small money, lite is the natural choice.

I means in, and ryggsäcken means the backpack.

So:

• i ryggsäcken = in the backpack

The definite form ryggsäcken is used because both speakers presumably know which backpack is meant. Swedish often uses the definite form where English also says the.

Because the sentence refers to a specific backpack, not just any backpack.

• i ryggsäcken = in the backpack
• i en ryggsäck would mean in a backpack, which sounds less specific

If the listener already knows which backpack is being talked about, Swedish naturally uses the definite form ryggsäcken.

Both are possible, but they are not exactly the same in emphasis.

• en flaska vatten = a bottle of water
• en vattenflaska = a water bottle

In many contexts, en flaska vatten is the most natural way to say you want to buy bottled water as a drink. It focuses on the contents: a bottle containing water.

By contrast, en vattenflaska can sometimes sound more like the container itself, depending on context.

Because in en flaska vatten, vatten works like a substance noun after the container noun flaska.

This is similar to English:

• a bottle of water
• not a bottle of a water

Swedish often leaves out the article in this type of expression:

• en kopp kaffe = a cup of coffee
• ett glas mjölk = a glass of milk
• en flaska vatten = a bottle of water

Åt mig means for me in the sense of doing something on someone’s behalf.

So:

• kan du köpa en flaska vatten åt mig? = can you buy a bottle of water for me?

With köpa, Swedish often uses åt when you mean buy something for someone.

Examples:

• Hon köpte en present åt sin vän. = She bought a present for her friend.
• Kan du köpa kaffe åt oss? = Can you buy coffee for us?

Sometimes, but åt mig is more natural here.

Both för and åt can relate to for, but åt is especially common when someone is doing or getting something on another person’s behalf, especially with verbs like köpa.

So in this sentence:

• köpa ... åt mig sounds very natural
• köpa ... för mig is less idiomatic here and may sound less standard to many speakers

A learner should strongly prefer åt mig with köpa in this kind of sentence.

The comma is common and helpful, especially when a subordinate clause comes first:

• Om du hittar lite växel i ryggsäcken, kan du köpa ...

It makes the sentence easier to read. In modern Swedish, punctuation can be a little less rigid than in English, but this comma is very normal and recommended.

Yes. The main clause is a question:

• kan du köpa en flaska vatten åt mig? = can you buy a bottle of water for me?

The initial om-clause just adds a condition:

• If you find some change in the backpack, can you buy a bottle of water for me?

So the whole sentence is still a question, and that is why it ends with a question mark.