memai ga suru toki ha, muri ni gaisyutusinai hou ga ii desu.

Because めまいがする is the standard Japanese expression for to feel dizzy / to have dizziness.

• めまい = dizziness, vertigo
• する = to do, but in many expressions it works more like to occur / to be felt

With physical sensations and symptoms, Japanese often uses noun + がする:

• 音がする = to hear a sound
• においがする = to smell something
• 咳がする = to have a cough
• めまいがする = to feel dizzy

So here, が marks the symptom being experienced.

とき means when or at the time when.

In this sentence:

• めまいがするとき = when you feel dizzy
• ときは adds は, which marks that time/situation as the topic

So めまいがするときは means something like:

• As for when you feel dizzy, ...
• When you feel dizzy, ...

The は gives it a general advice / topic-setting feeling.

The は highlights the situation being discussed.

Compare:

• めまいがするとき、 = when you feel dizzy, ...
• めまいがするときは、 = as for when you feel dizzy, ...

Using は makes it sound more like:

• In the case where you feel dizzy...
• When that happens...

It is very common in advice, instructions, and general statements.

無理に means forcibly, unreasonably, too hard, or against one’s limits.

Here it does not mean physical force exactly. It means doing something even though you should not, or pushing yourself.

So:

• 無理に外出しない = don’t force yourself to go out
• don’t go out if it means overexerting yourself

It adds the nuance that the person might be tempted to go out anyway, but should not push themselves.

Because 無理に is the adverb form.

• 無理 is a noun / na-adjective meaning impossibility, strain, or unreasonable effort
• adding に turns it into an adverb-like expression: unreasonably / forcibly / by pushing oneself

Similar pattern:

• 静か → 静かに = quietly
• 丁寧 → 丁寧に = carefully, politely
• 無理 → 無理に = by force / excessively / against one’s limits

外出する means to go out, especially to leave the house / go out somewhere.

So:

• 外出する = to go out
• 外出しない = not go out

This is the negative form of the verb する attached to the noun 外出.

Breakdown:

• 外出 = going out, outing
• する = to do
• 外出する = to go out
• 外出しない = to not go out

Because 〜ないほうがいい means it is better not to...

This pattern is very common for giving advice:

• 行かないほうがいい = you’d better not go
• 食べないほうがいい = you’d better not eat it
• 外出しないほうがいい = you’d better not go out

So here the advice is negative: not going out is the better choice.

〜ほうがいい literally means something like the side/option of doing X is good, but in natural English it usually means:

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

There are two common patterns:

• verb dictionary form + ほうがいい
  → better to do
• verb nai-form + ほうがいい
  → better not to do

Examples:

• 早く寝るほうがいい = You should go to bed early.
• そんなことは言わないほうがいい = You shouldn’t say that.

In your sentence:

• 外出しないほうがいいです = It’s better not to go out.

It is usually advice, not a direct command.

So 外出しないほうがいいです sounds like:

• You should not go out
• It’s better not to go out
• You’d better not go out

It is softer than an outright prohibition such as:

• 外出してはいけません = You must not go out.
• 外出しないでください = Please don’t go out.

So this sentence sounds caring and practical rather than harsh.

Because いいです is the polite form.

• いい = good
• いいです = is good / polite ending

In 〜ほうがいいです, the です makes the advice polite and appropriate for general instructions, announcements, medical advice, etc.

Compare:

• 外出しないほうがいい。 = plain style
• 外出しないほうがいいです。 = polite style

Not necessarily. It can mean:

• when you feel dizzy
• if you are experiencing dizziness
• in cases where you feel dizzy

So it can refer to the general situation, not only one precise second.

In health advice, Japanese often uses ときは this way to mean in that condition / in that situation.

Yes, that is a very natural translation.

Even though the Japanese structure is literally closer to:

• When you feel dizzy, it is better not to go out forcibly / by pushing yourself

natural English often becomes:

• If you feel dizzy, you shouldn’t force yourself to go out.
• When you feel dizzy, it’s better not to go out.
• If you’re dizzy, don’t push yourself to go out.

All of those capture the meaning well.

Semantically, 無理に describes the action 外出する: to go out by forcing yourself.

So the idea is:

• 無理に外出する = to force yourself to go out
• 無理に外出しないほうがいい = it’s better not to force yourself to go out

Even though the whole verb phrase is in the negative, 無理に still belongs with the action of going out.

Yes, it is often written as 目眩 or 眩暈, but in everyday Japanese it is very commonly written in hiragana as めまい.

Learners will often see hiragana more often because:

• the kanji are not very common
• the hiragana spelling is easy to read
• medical or general guidance often prefers accessible writing

So めまいがする in hiragana is completely normal.