taityou ga warui toki ha, muri ni kaisya ni ikanakute mo yoi desu.

In 体調が悪い, 体調 is the thing being described as bad, so が marks it as the subject of 悪い.

• 体調が悪い = one’s physical condition is bad
• Here, that whole clause then modifies とき: 体調が悪いとき = when one’s physical condition is bad

You could use は in other contexts, but 体調が悪い is the very standard way to say feel unwell / be in bad physical condition.

In Japanese, a plain-form adjective can directly modify a noun.

So:

• 悪い = bad
• とき = time / when
• 悪いとき = the time when it is bad / when it’s bad

And in the full sentence:

• 体調が悪いとき = when your physical condition is bad

This is very normal Japanese grammar. It is similar to:

• 忙しいとき = when busy
• 暇なとき = when free
• 行くとき = when going

The は marks the whole time phrase as the topic or frame of the sentence.

• 体調が悪いときは、... = As for when you’re feeling unwell, ... / When you’re feeling unwell, ...

Without は, the sentence could still be grammatical in some contexts, but ときは is very common when setting up a condition or situation and then saying what applies in that case.

無理に comes from 無理, which often means:

• unreasonable
• impossible
• forcing something
• overdoing it

With に, it becomes adverbial:

• 無理に = forcibly, by pushing yourself, unreasonably, beyond your limits

So in this sentence, it means something like:

• don’t force yourself to go
• don’t push yourself to go

It does not just mean physical force. It often refers to mental or physical strain too.

Because 行く often takes に to mark the destination.

• 会社に行く = go to the company / go to the office / go to work

Here 会社 is the place you are going to, so に is natural.

You may also see へ with 行く:

• 会社へ行く

That also means go to the office/company.
The difference is small here:

• に focuses a bit more on the destination
• へ focuses a bit more on direction

In everyday speech, に is very common.

This is a very important pattern:

• V-なくてもよい = do not have to V
• V-なくてもいい = same meaning, more casual/common

So:

• 行く = to go
• 行かない = not go
• 行かなくて = negative -te form
• 行かなくてもよいです = it is fine even if you do not go = you don’t have to go

Literally, it is close to:

• Even if you don’t go, that is good/fine.

But naturally, it means:

• You don’t need to go
• You don’t have to go

No. That is a very important distinction.

• 行かなくてもよいです = you don’t have to go
• 行ってはいけません = you must not go

So this sentence is giving permission not to go, not forbidding going.

Compare:

• 今日は行かなくてもいいです。 = You don’t have to go today.
• 今日は行ってはいけません。 = You must not go today.

Yes. These are common alternatives.

• 行かなくてもよいです = a bit more formal/stiff
• 行かなくてもいいです = very common
• 行かなくていいです = very common in speech
• 行かないでもいいです = also possible

All of these can mean you don’t have to go.

Among them, 行かなくてもいいです and 行かなくていいです are especially common in everyday conversation.

よい and いい are basically the same word.

• いい is the more common everyday form
• よい often sounds a bit more formal, careful, or written

In grammar patterns like 〜てもよい, learners often first encounter よい, because textbooks like slightly more neutral/formal forms.

So:

• 行かなくてもよいです = correct
• 行かなくてもいいです = also correct, and very natural

Because in Japanese, politeness is usually shown at the end of the main sentence, not in every part of it.

Here:

• 体調が悪いとき is a subordinate phrase modifying とき
• plain forms are normally used before nouns like とき
• the main sentence ends with よいです, which makes the whole sentence polite

So this mixture is completely normal:

• 体調が悪いときは、...よいです。

You would not normally say 体調が悪いですとき. That is ungrammatical.

The subject is omitted because Japanese often leaves out information that is understood from context.

In this sentence, the meaning is something like:

• When you are feeling unwell, you do not have to force yourself to go to work.
• or more generally, When one is feeling unwell, one does not have to force oneself to go to work.

Japanese avoids pronouns like you when they are not necessary. That is very natural.

Yes, literally it is go to the company/office.

But in Japanese, going to the company often naturally implies going to work, just as in English go to the office can imply go to work.

So in this sentence:

• 会社に行かなくてもよいです literally = you don’t have to go to the company
• natural meaning = you don’t have to go to work

The best translation depends on context, but go to work is very natural here.

Yes, it is grammatical and natural, especially in a careful or somewhat formal style.

It means something like:

• When you’re feeling unwell, you don’t have to force yourself to go to work.

In everyday conversation, a speaker might also say something slightly more casual, such as:

• 体調が悪いときは、無理して会社に行かなくていいです。
• 体調が悪いときは、無理に会社に行かなくてもいいです。

But the original sentence is perfectly good Japanese.