אני לא רוצה לרדת במדרגות כשכואבת לי הברך.

After רוצה (want), Hebrew normally uses an infinitive for the next verb, just like English uses to + verb:

• אני רוצה לרדת = I want to go down
• not usually אני רוצה יורד

So לרדת means to go down / to descend, and it is the correct form after רוצה.

In everyday unpointed Hebrew, רוצה is spelled the same for both masculine and feminine singular.

• masculine pronunciation: rotze
• feminine pronunciation: rotza

So from spelling alone, you usually cannot tell the speaker's gender here.

Yes, ב־ often means in, at, or with, but with movement expressions it can also describe the means or route.

So:

• לרדת במדרגות = to go down by way of the stairs / to go down the stairs

This is a very natural Hebrew way to say it. It does not mean to go down inside the stairs.

You may also hear לרדת את המדרגות in spoken Hebrew, but לרדת במדרגות is very common and natural.

כש means when. It is a short form of כאשר in many contexts.

In Hebrew, little function words like this are often written attached to the following word:

• כשכואבת = when it hurts / when ... hurts

So כשכואבת לי הברך means when my knee hurts.

Because the subject is הברך (the knee), and ברך is a feminine noun in Hebrew.

So the verb must agree with it:

• הברך כואבת = the knee hurts
• not הברך כואב

That is why the sentence has כואבת.

לי literally means to me, but in this kind of sentence it marks the person who is experiencing the pain.

Hebrew often expresses body pain like this:

• כואב לי הראש = my head hurts
• כואבת לי הברך = my knee hurts

So the structure is more literally something like:

• the knee hurts to me

But in natural English, we translate it as my knee hurts.

With body parts, Hebrew very often uses:

• the body part
  • dative pronoun (לי, לך, לו...)

So instead of saying my knee, Hebrew commonly says something equivalent to the knee hurts to me.

That is why הברך has the definite article ה־:

• כואבת לי הברך = my knee hurts

You can say things like הברך שלי, but in pain expressions the לי + ה־noun pattern is extremely common and idiomatic.

Yes, כשהברך כואבת לי is also possible.

Hebrew often allows more than one natural word order, especially in clauses like this. The given version:

• כשכואבת לי הברך

starts with the verb, which is very common in Hebrew and sounds quite natural.

Both versions mean basically the same thing:

• כשכואבת לי הברך
• כשהברך כואבת לי

The first may sound a little more flowing or idiomatic in many contexts.

In the present tense, Hebrew verbs do not show person as clearly as English verbs do, so subject pronouns are often stated.

For example, רוצה by itself means want/wants depending on context, so אני helps make it clearly I want.

• אני לא רוצה = I don't want

In context, native speakers sometimes omit אני, but here including it is completely normal and often preferable for clarity.

In Hebrew, לא usually goes directly before the verb or predicate it negates.

So:

• אני לא רוצה = I do not want

This is the standard way to make a sentence negative in Hebrew.

Yes, הברך שלי כואבת is grammatically correct and means my knee hurts.

But כואבת לי הברך is often more idiomatic in everyday Hebrew when talking about physical discomfort or pain.

A rough comparison:

• כואבת לי הברך = very natural for reporting pain
• הברך שלי כואבת = also correct, but a bit more explicit or neutral in structure

So the sentence you were given uses the more common Hebrew-style pain expression.

Here לרדת means to go down / descend, specifically to go down the stairs.

Hebrew לרדת is a general verb for downward movement:

• go down a hill
• go downstairs
• descend

In this sentence, במדרגות tells you that the downward movement is happening via the stairs.