Эти кроссовки мне подходят, потому что они не натирают пятку.

Because кроссовки is plural, so the demonstrative has to be plural too.

• это = this / it / that in some uses, but not the correct form here
• эти = these

So:

• эти кроссовки = these sneakers

Grammatically, эти is nominative plural, matching кроссовки.

Кроссовки is the normal word when talking about sneakers as footwear you wear as a pair.
The singular кроссовка does exist, but you usually only use it when talking about one shoe.

So:

• кроссовки = sneakers / trainers
• одна кроссовка = one sneaker

In everyday speech, the plural is much more common, just as English usually says shoes or sneakers rather than focusing on one shoe.

Because the verb подходить in this meaning works with the person in the dative case.

Russian structures this idea more like:

• These sneakers suit/fit to me

So:

• мне = to me / for me (dative of я)
• подходят = fit / suit / are suitable

That is why мне подходят means fit me / suit me, not I fit.

Because the subject is кроссовки, which is plural.

So the verb has to agree with it:

• кроссовки подходят = the sneakers fit
• кроссовка подходит = the sneaker fits

The same is true for они не натирают later in the sentence: они refers to the sneakers, so the verb is plural.

Not only size. Подходить can mean fit, suit, or be suitable depending on context.

With clothes or shoes, it often means something like:

• the size is right
• the shape works
• they are comfortable enough
• they suit the wearer

So in this sentence, мне подходят is broader than just they are the right size. It suggests these sneakers work well for me.

Literally, натирать means to rub, and with shoes it means to rub the skin painfully, often causing irritation or a blister.

So:

• натирать пятку = to rub/chafe the heel
• natural English translation: rub my heel raw, chafe my heel, or simply hurt my heel by rubbing

This is a very common thing to say about uncomfortable shoes.

Because пятка is the direct object of натирать.

Dictionary form:

• пятка = heel

After the verb, it becomes accusative singular:

• натирать пятку

So:

• nominative: пятка
• accusative: пятку

This is normal for a direct object in Russian.

Russian often uses the singular for a body part when speaking generally about the affected area.

So не натирают пятку can mean something like:

• they don’t rub my heel
• they don’t chafe at the heel

It does not necessarily mean only one heel in a literal, strict sense.

You can also hear the plural пятки if the speaker wants to emphasize both heels:

• они не натирают пятки

Both are possible; the singular here is natural.

Yes, you absolutely could say они мне не натирают пятку.

The sentence leaves out мне in the second clause because it is already clear whose heel is meant. Russian often omits words that are obvious from context.

So both are possible:

• ...потому что они не натирают пятку
• ...потому что они мне не натирают пятку

The second version is a bit more explicit.

Not always. Russian often drops subject pronouns when the subject is clear from the context.

So you could say:

• Эти кроссовки мне подходят, потому что они не натирают пятку.
• Эти кроссовки мне подходят, потому что не натирают пятку.

Both are understandable. Including они makes the subject extra clear and can sound slightly more complete or balanced.

No, Russian word order is flexible. The version you have is neutral and natural, but other orders are possible for emphasis.

For example:

• Эти кроссовки мне подходят... = neutral: These sneakers fit me...
• Мне подходят эти кроссовки... = emphasis on for me
• ...потому что пятку они не натирают = emphasis on heel

Russian word order often changes the focus, not the basic meaning.

A helpful stressed version is:

Э́ти кроссо́вки мне́ подходя́т, потому́ что они́ не натира́ют пя́тку.

A few notes:

• эти = EH-ti
• кроссо́вки = stress on со
• подходя́т = stress on the last я
• потому́ что = stress on му
• натира́ют = stress on ра
• пя́тку = stress on пя

In normal speech, что in потому что is often pronounced closer to што.