Пожалуйста, покажите мне сумочку, у которой короткий ремешок.

Покажите is the polite or plural imperative form of показать (to show).

• покажи = show! (to one person, informal)
• покажите = show! (to several people, or to one person politely/formally)

In a shop, when speaking to a salesperson, покажите is the normal polite choice.

Also, показать is a perfective verb, so покажите suggests a single complete action: Please show me...

Because мне is the dative case of я (I), and with показать / показывать the person something is shown to often goes in the dative.

So:

• покажите мне сумочку = show me the handbag

Compare:

• я = I
• меня = me (accusative/genitive, depending on use)
• мне = to me (dative)

This is very common in Russian:

• дайте мне... = give me...
• скажите мне... = tell me...
• покажите мне... = show me...

Because сумочку is the accusative singular form, and here it is the direct object of покажите.

Dictionary form:

• сумочка

In the sentence:

• покажите мне сумочку = show me a/the handbag

Since сумочка is a feminine noun ending in -а, the accusative singular changes to -у:

• сумочка → сумочку

Сумочка is a diminutive form of сумка.

Very roughly:

• сумка = bag
• сумочка = small bag / handbag / purse-like bag

Diminutives in Russian can suggest:

• smaller size
• affection
• softness/politeness
• a more specific everyday object

Here сумочка sounds very natural for a lady’s handbag or a small fashionable bag.

This is one of the most important parts of the sentence.

у которой короткий ремешок literally means something like:

• at which there is a short strap

But in natural English, it means:

• which has a short strap

Russian often expresses possession with у + a noun/pronoun in the genitive instead of using a verb like to have.

For example:

• У меня есть книга. = I have a book.
  (literally: At me there is a book.)

So:

• сумочку, у которой короткий ремешок = the handbag that has a short strap

Because которой refers back to сумочку / сумочка, which is:

• feminine
• singular

The word который changes form depending on:

1. the noun it refers to
2. the grammatical role inside the relative clause

Here it refers to сумочка, so it must be feminine singular.

But it also comes after у, and у requires the genitive case.

So:

• base form: которая (feminine singular nominative)
• after у: у которой (genitive feminine singular)

That is why you get которой.

Because inside the relative clause, ремешок is the thing that exists/is possessed.

The structure is like:

• у которой короткий ремешок

This works like the possession pattern:

• у неё короткий ремешок = she/it has a short strap

In this kind of construction:

• the possessor goes after у in the genitive
• the possessed thing is usually in the nominative

So:

• у которой = of which / which has
• короткий ремешок = a short strap

Because короткий agrees with ремешок, and here ремешок is nominative singular masculine.

So the adjective must also be nominative singular masculine:

• короткий ремешок

If ремешок were in another case, the adjective would change too. But in this sentence, the possession structure keeps ремешок in the nominative.

Ремешок means strap and is a diminutive form related to ремень (belt, strap).

In the context of a handbag:

• ремешок = strap
• often a smaller, thinner strap than a ремень

So короткий ремешок is a very natural way to say a short strap on a handbag.

Yes, absolutely.

A very common alternative is:

• Пожалуйста, покажите мне сумочку с коротким ремешком.

This literally means:

• Please show me a handbag with a short strap.

In many everyday situations, this version may even sound simpler and more natural.

The version with у которой is also correct, but it is a bit more like:

• the handbag which has a short strap

So both work:

• сумочку, у которой короткий ремешок
• сумочку с коротким ремешком

Because у которой короткий ремешок is a relative clause modifying сумочку.

Russian normally separates relative clauses with a comma.

So:

• сумочку, у которой короткий ремешок

This is similar to English:

• the handbag, which has a short strap or
• the handbag that has a short strap

Russian punctuation is very consistent here: if you have a clause introduced by который and it modifies a noun, you usually need a comma.

Yes, Russian word order is fairly flexible, though some versions sound more natural than others.

Original:

• Пожалуйста, покажите мне сумочку, у которой короткий ремешок.

You could also hear:

• Покажите, пожалуйста, мне сумочку, у которой короткий ремешок.
• Покажите мне, пожалуйста, сумочку, у которой короткий ремешок.

All of these are understandable.

The original version sounds polite and natural. Putting пожалуйста at the beginning is very common in requests.

In this sentence, yes: пожалуйста means please.

But in Russian it can also mean:

• you’re welcome
• go ahead
• here you are

So it is a very flexible politeness word.

Here:

• Пожалуйста, покажите мне... = Please show me...

That is the standard polite request meaning.