В магазине мне понравился клетчатый плед, а сестра взяла праздничную скатерть.

Because the verb понравиться works differently from English to like.

In Russian, понравиться literally means something like to please someone. So the person who experiences the feeling goes in the dative case:

• мне = to me
• тебе = to you
• ему/ей = to him/her

So:

• Мне понравился плед = I liked the blanket
• literally: The blanket pleased me

If you say я понравился, that means I was pleasing / I was liked, usually by another person:

• Я понравился сестре = My sister liked me (if the speaker is male)

So мне is correct because the blanket is the thing causing the feeling.

Because it agrees with плед, which is a masculine singular noun.

In the past tense, Russian verbs usually agree in gender and number with the subject:

• плед понравился — masculine singular
• скатерть понравилась — feminine singular
• письмо понравилось — neuter singular
• вещи понравились — plural

In this sentence, the thing that pleased the speaker is клетчатый плед, so the verb is:

• понравился — masculine singular

Even though English says I liked, Russian is structured more like:

• To me, the plaid blanket pleased

That is why the verb matches плед, not мне.

В магазине is in the prepositional case because it answers where?

• в
  • prepositional often means in / at
• магазин → в магазине

So:

• магазин = store/shop
• в магазине = in the store / at the store

Compare:

• в магазин = to the store (direction, accusative)
• в магазине = in the store (location, prepositional)

Here the sentence describes where the action happened, so в магазине is correct.

Because the two noun phrases are in different cases.

1. клетчатый плед

This is the thing that pleased the speaker, so it is the grammatical subject of понравился. It stays in the nominative case:

• клетчатый — masculine nominative singular
• плед — masculine nominative singular

2. праздничную скатерть

This is the direct object of взяла (took/chose), so it goes in the accusative case:

• праздничная → праздничную
• скатерть stays скатерть because it is an inanimate feminine noun of this pattern, and its accusative singular looks the same as the nominative

So the difference comes from grammar:

• клетчатый плед = subject, nominative
• праздничную скатерть = object, accusative

Because adjectives in Russian must agree with the noun they describe in:

• gender
• number
• case

The noun скатерть is:

• feminine
• singular
• here in the accusative case

So the adjective праздничный changes to match it:

• masculine nominative: праздничный
• feminine nominative: праздничная
• feminine accusative: праздничную

That is why we get:

• праздничную скатерть

This agreement is one of the most important patterns in Russian grammar.

In this sentence, а links the two parts with a slight contrast or comparison:

• В магазине мне понравился клетчатый плед, а сестра взяла праздничную скатерть.

This feels like:

• I liked a plaid blanket, while my sister chose a festive tablecloth
• or ..., and my sister, on the other hand, took...

So а often means:

• and (with contrast)
• while
• whereas
• sometimes almost but

By contrast, и would sound more like simple addition:

• ... и сестра взяла ... = ... and my sister took ...

Here а is natural because it compares my reaction with my sister’s action.

Yes, взяла literally means took, but in shopping contexts it can often mean things like:

• chose
• picked
• decided on
• sometimes even bought, depending on context

So сестра взяла праздничную скатерть could be understood as:

• my sister took a festive tablecloth
• my sister chose a festive tablecloth

Also notice that взяла is:

• past tense
• feminine singular

because сестра is feminine.

The infinitive is взять, which is a perfective verb, so it suggests a completed action: she made the choice / took it.

Because in the Russian past tense, verbs agree with the subject in gender and number.

The subject is сестра, which is feminine singular, so the verb is feminine singular too:

• взял — he took
• взяла — she took
• взяло — it took
• взяли — they took

So:

• сестра взяла = the sister took / my sister took

This is a very common feature of Russian past tense verbs.

Russian often omits possessive words like my, your, or his when the meaning is already clear from context.

So:

• сестра can mean sister
• in context, it often naturally means my sister

In English, we usually need my sister, but Russian is often comfortable just saying сестра if there is no confusion.

If you want to be explicit, you could say:

• моя сестра взяла праздничную скатерть

But in normal Russian, leaving out моя is very common.

Because понравиться usually refers to a reaction to something as a whole, often a single completed impression: to like, to find pleasing.

Here the speaker saw the blanket in the store and had that reaction, so понравился fits well.

Compare:

• мне понравился плед = I liked the blanket / the blanket appealed to me
• мне нравился плед = I used to like the blanket / I was liking the blanket in an ongoing or background sense
• мне нравится плед = I like the blanket (present tense, general statement)

So in this sentence, понравился is natural because it describes a completed past reaction in the store.

Russian word order is flexible, but the version in the sentence is natural and clear.

Current order:

• В магазине мне понравился клетчатый плед, а сестра взяла праздничную скатерть.

This sounds smooth and neutral:

• first the setting: in the store
• then the speaker’s reaction
• then the sister’s action

You could change the order for emphasis, for example:

• Мне в магазине понравился клетчатый плед...
• Клетчатый плед мне понравился в магазине...

These are still grammatical, but they shift the focus slightly.

So the word order can move, but the original version is a very natural standard way to say it.