hiza ga itai toki ha, muri ni hasiranai hou ga ii desu.

ひざが痛い is a very common Japanese pattern for talking about body symptoms.

• ひざ = knee
• が marks the thing experiencing the condition
• 痛い = painful / hurts

So ひざが痛い literally means the knee hurts or my knee hurts.

With body parts and sensations, Japanese often uses が this way:

• 頭が痛い = my head hurts
• お腹が痛い = my stomach hurts
• 目がかゆい = my eyes are itchy

In English, we often say my knee hurts, but in Japanese the my is usually omitted when it is obvious from context.

Japanese often leaves out information that is clear from context. In a sentence about someone’s physical condition, ひざが痛い is naturally understood as my knee hurts, your knee hurts, or their knee hurts, depending on the situation.

For example:

• At the doctor: ひざが痛いです usually means my knee hurts
• Giving advice to someone: ひざが痛いときは… can mean when your knee hurts...

This omission is very normal in Japanese and does not sound incomplete.

とき means time or when, and は marks that whole phrase as the topic.

So:

• ひざが痛いとき = when your knee hurts
• ひざが痛いときは = as for when your knee hurts / when your knee hurts, ...

The は gives a sense of speaking generally about that situation.

Compare:

• ひざが痛いとき、無理に走らないほうがいいです。
  = When your knee hurts, it’s better not to force yourself to run.
• ひざが痛いときは、無理に走らないほうがいいです。
  = As for times when your knee hurts, it’s better not to force yourself to run.

The second feels a bit more like general advice.

When a noun like とき is modified, Japanese puts the modifying phrase before it.

So:

• ひざが痛いとき = the time when the knee hurts

This is the same basic pattern as:

• 忙しいとき = when busy
• 行くとき = when going / before going
• 帰ったとき = when I returned / when someone returned

You can think of it as [clause] + とき = when [clause].

無理 means things like unreasonable, too much, forcing oneself, or overdoing it.
With に, it becomes adverbial:

• 無理に = forcibly, by pushing yourself, beyond what is reasonable

In this sentence, 無理に走る means something like:

• to run while pushing yourself
• to force yourself to run
• to overdo it by running

So this is not just running in general. It specifically suggests running in a way that strains you despite the pain.

Because the sentence uses the pattern ～ないほうがいい, which means it’s better not to...

So:

• 走る = run
• 走らない = not run
• 走らないほうがいい = it’s better not to run

This pattern is very common for giving advice:

• 今日はお酒を飲まないほうがいいです。 = You’d better not drink today.
• そんなに心配しないほうがいいです。 = It’s better not to worry so much.

Literally, ほう means side or option.

So 走らないほうがいい literally means:

• the not-running option is good

That sounds unnatural in English, but in Japanese it is a standard way to say:

• it’s better not to run
• you should probably not run

This pattern compares two possible choices, even if only one is stated:

• run
• not run

Japanese says the not running side is the better one.

It is usually advice, not a strict command.

～ほうがいいです often means:

• it’s better to...
• you should...
• you’d better...

So 無理に走らないほうがいいです sounds like considerate advice.

It is softer than stronger prohibitions like:

• 走ってはいけません = you must not run
• 走らないでください = please do not run

So this sentence does not sound harsh. It sounds more like a recommendation based on someone’s condition.

Here, に turns a noun-like word into an adverbial expression.

• 無理 = unreasonable effort / forcing oneself
• 無理に = forcibly / by overdoing it / against reason

This is similar to other adverbial uses of に:

• 静かに話す = speak quietly
• 丁寧に書く = write carefully/politely
• 簡単にできる = can do easily

So 無理に走る means to run in a forced or excessive way.

Yes, but the nuance would change.

• ひざが痛いときは = when your knee hurts / in situations where your knee hurts
• ひざが痛いなら = if your knee hurts / if it’s the case that your knee hurts

ときは focuses on the time or situation.
なら focuses more on a condition or if that is true.

So:

• ひざが痛いときは、無理に走らないほうがいいです。
  sounds like general advice for that circumstance.
• ひざが痛いなら、無理に走らないほうがいいです。
  sounds more like responding to the condition: if your knee hurts, then...

Both are natural, but ときは fits general health advice very well.

です makes the sentence polite.

The core grammar is:

• 無理に走らないほうがいい = it’s better not to force yourself to run

Adding です makes it appropriate for normal polite speech:

• 無理に走らないほうがいいです

This is the standard style for giving polite advice.

A casual version would be:

• ひざが痛いときは、無理に走らないほうがいい。

It can depend on context.

ひざ literally means knee, but Japanese often does not mark singular vs. plural the way English does. So it may mean:

• one knee
• your knee area
• your knees in general

In a sentence like this, the exact number usually does not matter. The point is simply if your knee is hurting.

No, it is not grammatically required, but it is very natural in writing.

• ひざが痛いときは無理に走らないほうがいいです。
• ひざが痛いときは、無理に走らないほうがいいです。

Both are correct.

The comma helps show a pause after the topic phrase ひざが痛いときは and makes the sentence easier to read. It is especially common when the first part of the sentence sets the situation and the second part gives advice or a conclusion.