Из-за растяжения мне пока трудно ходить быстро, но врач говорит, что через неделю станет лучше.

Из-за means because of, due to, or as a result of. It takes the genitive case.

So:

• из-за растяжения = because of the sprain / due to the strain

A useful nuance: из-за often introduces a cause with a negative result, so it fits very naturally here.

Because из-за requires the genitive case.

The dictionary form is:

• растяжение = sprain, strain, stretching

Its genitive singular form is:

• растяжения

So:

• из-за растяжения = because of a sprain/strain

In this sentence, растяжение most naturally means a sprain or muscle/ligament strain.

The word can have broader meanings in other contexts, such as stretching or extension, but here the medical context and the fact that walking is difficult make sprain/strain the intended meaning.

Russian often uses an impersonal construction with words like трудно, легко, жарко, холодно, etc.

The pattern is:

• кому? + трудно + infinitive

So:

• мне трудно ходить = it is hard for me to walk

Here мне is in the dative case because it marks the person experiencing the difficulty.

This is different from:

• я не могу ходить быстро = I can’t walk quickly

That would emphasize inability.
Мне трудно ходить быстро means more it’s difficult / hard for me to walk quickly, which is a bit softer and more natural here.

Here трудно is not an adjective like трудный. It is a predicative word (often called a category of state word in Russian grammar).

It is used in sentences like:

• Мне трудно = It’s hard for me
• Ему легко = It’s easy for him
• Нам холодно = We are cold

So трудно stays the same; it does not change for gender, number, or case in this use.

Here пока means for now, so far, or at the moment.

So:

• мне пока трудно ходить быстро = for now, it’s hard for me to walk quickly

It suggests the situation is temporary, which matches the second half of the sentence: the doctor says it will improve.

In other contexts, пока can also mean while or even bye, so context matters a lot.

This is a very common question.

• идти usually refers to going/walking in one specific direction, at one specific time
• ходить usually refers to walking in general, being able to walk, walking as an activity, or repeated/multiple-direction movement

Here the meaning is about general ability:

• трудно ходить быстро = it is hard to walk quickly

It is not about one specific trip from one place to another, so ходить is the better choice.

If you said мне трудно идти быстро, it would sound more like it is hard for me to go/walk quickly right now in this particular instance.

Yes, both are possible:

• ходить быстро
• быстро ходить

Both mean to walk quickly.

In this sentence, ходить быстро sounds very natural and neutral.
Putting быстро before the verb can slightly emphasize quickly more, but the difference is small here.

So the original word order is completely normal.

Yes. The clause

• мне пока трудно ходить быстро

is an impersonal construction.

There is no grammatical subject like я. Instead, Russian expresses the situation as:

• to me
  • it is difficult
    • to walk quickly

This is very common in Russian and often sounds more natural than trying to force a personal subject into the sentence.

Russian punctuation requires both commas here.

1. Before но
   Но means but, and it joins two clauses, so a comma is needed:

• ..., но врач говорит...

2. Before что
   Что introduces a subordinate clause, like that in English:

• врач говорит, что... = the doctor says that...

Russian writes this comma very consistently, even where English might sometimes omit that.

Что introduces reported speech or a statement, like English that.

So:

• врач говорит, что через неделю станет лучше
• the doctor says that it will get better in a week

It connects the main clause:

• врач говорит = the doctor says

with the content of what the doctor says.

Both are possible in some contexts, but станет лучше is especially natural here because it emphasizes change:

• станет лучше = will become better / will get better
• будет лучше = will be better

Since the person is currently having difficulty and the doctor predicts improvement, Russian often uses стать + comparative:

• станет лучше
• станет легче
• станет хуже

This highlights that the condition will change.

Через неделю means in a week or a week from now.

The preposition через here is used with the accusative case to show that something happens after a period of time has passed.

So:

• через неделю = in a week
• через два дня = in two days
• через месяц = in a month

It does not mean during the week. It means after one week.

Russian often leaves this kind of subject implicit when it is obvious from context.

Here станет лучше means something like:

• it will get better
• things will improve
• your condition will improve

English uses a dummy subject it, but Russian often does not need one.

If you wanted, you could make the person more explicit:

• мне через неделю станет лучше = I will feel better in a week

But the shorter через неделю станет лучше is very natural after talking about the injury and difficulty walking.