Während ich die Hose anprobiere, prüft sie die Passform.

Because während introduces a subordinate clause.

In German, subordinate clauses normally send the conjugated verb to the end:

• Ich probiere die Hose an. = main clause
• Während ich die Hose anprobiere, ... = subordinate clause

So after während, you do not keep normal main-clause word order.

A useful pattern is:

• Während + subject + other elements + verb

Example:

• Während ich esse, liest er.
• Während sie arbeitet, telefoniere ich.

Because anprobieren is a separable verb.

In a normal main clause, separable verbs split:

• Ich probiere die Hose an.

But in a subordinate clause, the verb usually stays together and goes to the end:

• Während ich die Hose anprobiere, ...

So:

• main clause: Ich probiere die Hose an.
• subordinate clause: ..., weil ich die Hose anprobiere.

This is very common with separable verbs in German.

Because the sentence begins with the subordinate clause:

• Während ich die Hose anprobiere, ...

After that whole clause, the main clause starts. In German main clauses, the finite verb must be in position 2.

So the structure is:

1. Während ich die Hose anprobiere = position 1
2. prüft = position 2
3. sie = subject after the verb

That is why you get:

• Während ich die Hose anprobiere, prüft sie die Passform.

If the main clause came first, you would say:

• Sie prüft die Passform, während ich die Hose anprobiere.

Here it means she.

Why?

• sie can mean she
• sie can also mean they

But the verb is prüft, which is singular.
If it meant they, the verb would be prüfen.

So:

• sie prüft = she checks
• sie prüfen = they check

Also, it is not the formal you, because that would be capitalized: Sie.

Die Hose is in the accusative case because it is the direct object of anprobieren.

You are trying on the trousers/pants, so the trousers/pants receive the action.

The noun Hose is feminine:

• nominative: die Hose
• accusative: die Hose

So the form does not change here, but the case is still accusative.

Compare with a masculine noun, where you would see a change:

• der Mantel → ich probiere den Mantel an

Yes. Die Passform is also accusative, because it is the direct object of prüfen.

She is checking the fit, so the fit is the thing being checked.

Passform is feminine, so again:

• nominative: die Passform
• accusative: die Passform

Just like die Hose, the article stays the same because the noun is feminine.

Passform means fit in the sense of how a piece of clothing fits the body.

It is a common word in clothing contexts.

Examples:

• Die Passform ist gut. = The fit is good.
• Die Jacke hat eine schlechte Passform. = The jacket has a poor fit.

It is more specific than a general verb like passen.
Passform is the noun: the fit, the cut, how well something fits.

Because während means during / while, showing that two actions happen at the same time.

In this sentence:

• I am trying on the pants
• she is checking the fit
• both happen simultaneously

So während is a very natural choice.

Wenn can mean when or if, and it is often used for repeated situations or conditions. It does not emphasize the idea of during the same time period as strongly.

Compare:

• Während ich die Hose anprobiere, prüft sie die Passform.
  = While I’m trying on the pants, she checks the fit.

• Wenn ich die Hose anprobiere, prüft sie die Passform.
  = When(ever) I try on the pants, she checks the fit.
  This can sound more like a repeated or general situation.

Yes, usually.

Anprobieren means to try on, especially for:

• clothes
• shoes
• accessories

Examples:

• Ich probiere die Jacke an.
• Sie probiert die Schuhe an.

Do not confuse it with:

• probieren = to try, to taste
• ausprobieren = to try out, test
• anprobieren = to try on

So for clothing, anprobieren is the correct verb.

Normally, no. For clothing, German uses anprobieren or probieren ... an, not just probieren.

• Ich probiere die Hose an. = correct
• Ich probiere die Hose. = not natural for I try on the pants

By itself, probieren often means:

• to taste something
• to try something in a more general sense

So with clothes, keep anprobieren.

In German, die Hose is usually singular.

That is different from English, where pants is grammatically plural.

So:

• die Hose = the pants / the pair of trousers
• die Hosen = the pants / trousers in the plural sense, meaning multiple pairs

Examples:

• Ich kaufe eine Hose. = I’m buying a pair of pants.
• Ich kaufe zwei Hosen. = I’m buying two pairs of pants.

This is a very common difference between German and English.

It is in the present tense.

• anprobiere = present tense, I try on / I am trying on
• prüft = present tense, she checks / she is checking

In German, the present tense often covers both:

• simple present
• present progressive

So this sentence can mean:

• While I try on the pants, she checks the fit.
• While I’m trying on the pants, she’s checking the fit.

German usually does not need a separate -ing form like English.

Yes.

You can also say:

• Sie prüft die Passform, während ich die Hose anprobiere.

That means the same thing.

The difference is mainly about focus:

• starting with Während ich die Hose anprobiere emphasizes the time setting first
• starting with Sie prüft die Passform emphasizes her action first

Both are correct.