Её лодыжка всё ещё болит, поэтому она носит удобные кроссовки.

Here её means her.

In Russian, его, её, and их are commonly used as possessive words meaning his, her, and their. Unlike adjectives such as мой / моя / моё / мои, they do not change to match the noun.

So you get:

• её лодыжка — her ankle
• её рука — her hand
• её ноги — her legs
• к её лодыжке — to her ankle

The form её stays the same in all of these.

Because лодыжка is the subject of болит.

Russian says this in a very similar way to English:

• Лодыжка болит — The ankle hurts / aches

The thing that hurts is the grammatical subject, so it appears in the nominative.

In the present tense, Russian verbs do not show gender. They only show person and number.

So:

• лодыжка болит — the ankle hurts
• нога болит — the leg hurts
• палец болит — the finger hurts

All of these use the same 3rd person singular form.

Gender matters in the past tense, not the present:

• лодыжка болела — the ankle hurt / was aching

This is a very common question.

• болит means hurts / aches / is sore, usually about a body part or something causing pain
• болеет usually means is ill / is sick, or sometimes supports a team

So:

• Её лодыжка болит — Her ankle hurts
• Она болеет — She is sick

Even though both come from болеть, the meanings are different depending on the subject and context.

Всё ещё means still.

So:

• Её лодыжка всё ещё болит = Her ankle still hurts

It is best learned as a fixed expression.

Because всё ещё is a set adverbial phrase. The всё here does not agree with лодыжка or any other noun.

So don’t think of it as a normal adjective or pronoun here. Just learn:

• всё ещё = still

Also, note that it is всё with ё, not все.

Yes — and that is very natural Russian.

Russian often expresses pain with the pattern:

• У кого? + болит + body part

So:

• У неё болит лодыжка — Her ankle hurts

The sentence you were given, Её лодыжка всё ещё болит, is also correct. It sounds a bit closer to English structure, while У неё болит лодыжка is often more idiomatic in everyday Russian.

Because the sentence has two clauses:

• Её лодыжка всё ещё болит
• поэтому она носит удобные кроссовки

The second clause gives the result or consequence, so a comma is normally used before поэтому.

Russian often does drop subject pronouns when the subject is obvious.

But here the subject changes:

• first clause: лодыжка is the subject
• second clause: она is the subject

If you removed она, the sentence would be less clear, because the reader has just been focused on лодыжка. So она is useful and natural here.

Because носит means wears, while надевает means puts on.

Compare:

• Она носит удобные кроссовки — She wears comfortable sneakers
• Она надевает удобные кроссовки — She puts on comfortable sneakers

In your sentence, the idea is her usual choice of footwear because of her ankle, so носит is the better verb.

Because it describes кроссовки, which is plural.

Russian adjectives agree with the nouns they describe in:

• gender
• number
• case

So:

• удобный кроссовок would not work here
• удобные кроссовки is correct because the noun is plural

Because кроссовки is the direct object of носит, so it is in the accusative plural.

However, кроссовки is an inanimate noun, and for inanimate plural nouns, the accusative plural looks the same as the nominative plural.

So:

• nominative plural: удобные кроссовки
• accusative plural: удобные кроссовки

That is why the form stays удобные.

In normal Russian, кроссовки is usually used in the plural, like English sneakers or trainers.

You usually talk about the pair as plural footwear:

• Она носит кроссовки — She wears sneakers

A singular form кроссовка can exist if you really mean one shoe, but that is not the normal everyday way to refer to this kind of footwear.

You may also hear:

• пара кроссовок — a pair of sneakers

Yes, Russian word order is flexible, but the original sentence is neutral and natural:

• Её лодыжка всё ещё болит, поэтому она носит удобные кроссовки.

You can change word order for emphasis, but not every order sounds equally natural. For example, the given order keeps the flow clear:

1. problem — her ankle still hurts
2. result — so she wears comfortable sneakers

So for a learner, the original order is a very good model to follow.