kanozyo no aoi mahuraa ha kooto ni yoku niaimasu.

彼女 means she / her / girlfriend, depending on context, and の links it to the next noun.

So 彼女の青いマフラー means her blue scarf.

In this sentence, の works like English 's or of:

• 彼女のマフラー = her scarf
• 私の本 = my book
• 先生の車 = the teacher’s car

Here, 彼女の modifies 青いマフラー.

青い is an i-adjective, and it must keep its adjective form when directly modifying a noun.

So:

• 青いマフラー = a blue scarf

You cannot say 青マフラー in normal grammar.

A quick comparison:

• 青い空 = blue sky
• 赤い車 = red car
• 白い猫 = white cat

The -い is part of the adjective form.

は marks the topic of the sentence.

Here, the topic is 彼女の青いマフラー — her blue scarf. The sentence is talking about that scarf and then saying something about it:

• 彼女の青いマフラーは… = As for her blue scarf...

Then the rest of the sentence tells us what is true about it:

• コートによく似合います = it suits the coat well

So the whole structure is:

• [Topic] は [comment]

This does not always match English subject structure exactly, but in this sentence the topic is also the thing being described.

With 似合う, the thing that something suits is often marked by に.

So:

• マフラーがコートに似合う = The scarf suits the coat
• more naturally in English: The scarf goes well with the coat

Here, に does not mean a simple to. It marks the item that something matches or looks good with.

More examples:

• この帽子はあなたに似合う。 = This hat suits you.
• 赤は彼女に似合う。 = Red suits her.

So in your sentence:

• コートに似合います = goes well with the coat / suits the coat

Here, よく means well, not often.

So:

• よく似合います = suits well / goes well together

This is a very common point of confusion because よく can mean different things depending on context:

• よく行く = go often
• よく見える = can see well
• よく似合う = suits well

In this sentence, it describes how well the scarf matches the coat.

This is one of the most common questions, because Japanese and English organize this idea differently.

In this sentence:

• 彼女の青いマフラーはコートによく似合います。

the topic is her blue scarf, and it is the thing that looks good with the coat.

So the basic idea is:

• The scarf suits the coat
• or more natural English: Her blue scarf goes very well with the coat

With 似合う, the thing that looks good is often the topic or subject, and the person/clothing/item it suits is marked with に.

Pattern:

• A は B に 似合う
• A suits B / A looks good on/with B

Examples:

• この色は彼に似合う。 = This color suits him.
• そのネクタイはスーツに似合う。 = That tie goes well with the suit.

In Japanese, マフラー usually means scarf, especially a warm scarf worn around the neck.

So even though English muffler may make you think of:

• a car muffler, or
• an old-fashioned word for a scarf,

in modern Japanese マフラー is a normal word for scarf.

So here:

• 青いマフラー = blue scarf

Both particles can sometimes appear with 似合う, but they do different jobs.

• は marks the topic
• が marks the subject more directly or can add focus

Here, は sounds natural because the sentence is about her blue scarf in a general descriptive way:

• 彼女の青いマフラーはコートによく似合います。
• As for her blue scarf, it goes well with the coat.

If you used が:

• 彼女の青いマフラーがコートによく似合います。

it would sound more like you are specifically identifying that scarf as the one that suits the coat, perhaps in contrast to other items.

So は is a natural choice when introducing the scarf as the topic of discussion.

Japanese does not have articles like English a / an / the.

So コート can mean:

• a coat
• the coat
• sometimes just coat / coats depending on context

The exact meaning comes from the situation, not from an article.

That is why the translation may say the coat even though Japanese does not have a separate word for the.

Yes. 似合います is the polite non-past form.

The dictionary form is:

• 似合う

So:

• 似合う = plain form
• 似合います = polite form

Examples:

• この服は彼に似合う。 = plain
• この服は彼に似合います。 = polite

In your sentence, the polite form makes it sound natural and standard.

Japanese word order is more flexible than English, but particles show the grammatical roles, so the original order is the most straightforward and natural.

Standard order here:

• 彼女の青いマフラーはコートによく似合います。

You might also hear variations for emphasis, but beginners should learn the normal pattern first:

• [topic] は [target] に [adverb] [verb]

So this sentence is a very good model:

• 彼女の青いマフラー = topic
• コートに = what it goes well with
• よく = well
• 似合います = suits / goes well with

Both are correct, but goes well with is often the more natural English translation in this sentence.

Why?

Because the sentence is about one clothing item matching another clothing item:

• Her blue scarf goes well with the coat.

If the target were a person, suits is often the most natural:

• その色は彼女に似合います。 = That color suits her.

So for this sentence:

• suits the coat = grammatically fine
• goes well with the coat = often more natural English

Both reflect the meaning of 似合います well.