memai ga aru toki ha, kyuu ni ugokanai hou ga ii to isya ni iwaremasita.

Yes, ある literally means to exist / to be, but in Japanese it is also very commonly used for conditions, symptoms, and experiences.

So めまいがある means:

• to have dizziness
• to feel dizzy
• literally, there is dizziness

This pattern is very common:

• 熱がある = I have a fever
• 咳がある = I have a cough
• 頭痛がある = I have a headache

So even though English often uses have, Japanese often uses X がある for symptoms.

The particle が marks what exists or what is present. Since ある is being used in the sense of there is dizziness, めまい is the thing that exists, so が is the natural particle.

• めまいがある = dizziness is present

Why not を?
Because ある does not normally take a direct object with を. It describes existence, not an action done to something.

Why not は?
You could say めまいはある in some contexts, but that would add contrast, like:

• 頭痛はないけど、めまいはある。 = I don’t have a headache, but I do have dizziness.

In this sentence, plain が is the neutral choice.

とき means when or at the time when.

• めまいがあるとき = when you have dizziness / when you feel dizzy

The は after it marks that whole time phrase as the topic:

• めまいがあるときは、... = As for when you feel dizzy, ... = When you feel dizzy, ...

So the structure is:

• [めまいがある とき] は
• As for the time when dizziness exists...

The は helps set up the condition or situation being discussed. It is very common in advice sentences.

急 by itself is a noun or a na-adjective meaning sudden or urgent. To modify a verb, it usually becomes 急に, which works adverbially.

So:

• 急な動き = a sudden movement
• 急に動く = to move suddenly

In your sentence:

• 急に動かないほうがいい = it is better not to move suddenly

So the に changes 急 into the form used to describe how the movement happens.

This is just the standard Japanese advice pattern.

• Vないほうがいい = had better not do V / it’s better not to do V
• Vたほうがいい = had better do V / it’s better to do V

So:

• 動かないほうがいい = it’s better not to move
• 休んだほうがいい = it’s better to rest

Japanese strongly prefers these patterns for giving practical advice.

Your alternative, 動くほうがよくない, is not the normal way to say it. It sounds awkward and unnatural.

Literally, ほう means side / direction / option, and ほうがいい means that option is good / better.

So:

• 動かないほうがいい = the option of not moving is better

This is why the pattern works with both positive and negative verbs:

• 早く寝たほうがいい = you should go to bed early
• 無理しないほうがいい = you shouldn’t overdo it

It often sounds like advice rather than a strict command.

No, it is usually softer than must not.

動かないほうがいい means something like:

• you’d better not move suddenly
• it’s better not to move suddenly
• you shouldn’t move suddenly

It is advice or recommendation. The exact strength depends on context.

In this sentence, because the advice comes from a doctor, it may feel fairly strong in real life, but grammatically the expression itself is still advice, not an absolute prohibition.

Stronger expressions would be things like:

• 動いてはいけない = you must not move
• 動かないでください = please don’t move

That と is the quotation particle. It marks what was said.

The quoted content is:

• めまいがあるときは、急に動かないほうがいい

Then:

• と 医者に言われました = I was told by the doctor that ...

So the sentence structure is:

• [quoted advice] と + 医者に + 言われました

Japanese uses と very often after speech, thoughts, and reported statements.

This is a very common question, because the English translation often just says The doctor said..., but the Japanese says something slightly different.

• 医者が言いました = the doctor said
• 医者に言われました = I was told by the doctor

Your sentence uses the passive form 言われました, so the focus is on the speaker receiving the advice.

That makes perfect sense here, because the meaning is:

• I was told by a doctor that when I feel dizzy, I shouldn’t move suddenly.

The particle に marks the source/agent in this passive sentence.

So:

• 医者が言いました focuses on the doctor as the speaker
• 医者に言われました focuses on the speaker as the person who received the message

Yes. It is the passive past polite form of 言う.

Step by step:

• dictionary form: 言う = to say
• passive: 言われる = to be told / to be said to
• polite past: 言われました = was told

So:

• 医者に言われました = I was told by the doctor

In many cases, English translates this naturally as the doctor told me, but the Japanese grammar is passive.

Japanese often leaves the subject unstated when it is understood from context.

Here, the full meaning is something like:

• When you/I have dizziness, it’s better not to move suddenly, the doctor told me.

Because of 医者に言われました, we know the speaker is reporting advice they received. So in natural English, it often becomes:

• The doctor told me that when I feel dizzy, I shouldn’t move suddenly.

The understood person inside the advice is usually the speaker, though in a more general context it could also be read as general advice for anyone.

It can mean both, depending on context, but in a sentence like this it is usually understood as a general condition:

• when you feel dizzy
• whenever you have dizziness

So it is not just one single moment. It can describe a repeated situation or a general rule.

That is very natural in Japanese:

• 熱があるときは、無理しないほうがいい。 = When you have a fever, you shouldn’t push yourself.

The comma is there to make the sentence easier to read and to show a pause after the topic/condition:

• めまいがあるときは、
• As for when you feel dizzy, ...

Japanese commas are often used more flexibly than English commas. This comma is not absolutely required, but it is very natural and helpful here because the sentence has a clear setup followed by the main advice.