Μην βγαίνεις έξω όταν βρέχει πολύ.

Because μην is the negative particle used for commands, prohibitions, and subjunctive forms.

• Δεν βγαίνεις έξω. = You are not going outside / You don't go outside
• Μην βγαίνεις έξω. = Don't go outside

So in this sentence, μην shows that the speaker is telling someone not to do something.

Yes, the form βγαίνεις looks the same as the 2nd person singular present indicative form, but after μην it functions as part of a negative subjunctive command.

In other words, Greek often uses the same verb form here, but the particle μην changes how it works:

• βγαίνεις by itself can mean you go out / you are going out
• μην βγαίνεις means don’t go out

So the meaning comes from the whole combination μην + verb, not from the verb form alone.

This is an important aspect difference.

• Μην βγαίνεις uses the imperfective form.
• Μην βγεις uses the perfective form.

Very roughly:

• Μην βγαίνεις έξω όταν βρέχει πολύ. = Don’t go outside when it’s raining heavily
  This sounds like general advice or something repeated/habitual.
• Μην βγεις έξω! = Don’t go outside!
  This often sounds more like a single action or one specific occasion.

So in your sentence, βγαίνεις is used because the idea is more general: whenever it rains a lot, don’t go outside.

Yes, βγαίνω already means go out / exit / leave, so technically έξω is not always strictly necessary.

But Greek very often says βγαίνω έξω, and it sounds completely natural. Έξω reinforces the idea of outside.

Compare:

• Μην βγαίνεις όταν βρέχει πολύ. = still understandable, more like don’t go out
• Μην βγαίνεις έξω όταν βρέχει πολύ. = clearly don’t go outside

So έξω makes the meaning more explicit and natural.

Όταν means when, and it works very naturally for repeated or general situations.

So:

• Μην βγαίνεις έξω όταν βρέχει πολύ. = Don’t go outside when it rains heavily

This means something like whenever that situation happens.

You may also hear:

• άμα βρέχει πολύ
• αν βρέχει πολύ

especially in speech, depending on style and region. But όταν is a very standard and safe choice here.

Greek uses impersonal verbs for weather, just like English often does with it rains.

• βρέχει = it rains / it is raining

But Greek does not need a separate subject word like English it. The verb by itself is enough.

This is very common with weather expressions:

• βρέχει = it’s raining
• χιονίζει = it’s snowing

So there is no missing word here; βρέχει is complete on its own.

Here πολύ modifies the verb βρέχει, so it means something like:

• a lot
• heavily

So βρέχει πολύ means it rains a lot or, more naturally in English here, it’s raining heavily.

This is different from very before an adjective, though πολύ can also mean very in other contexts:

• πολύ καλός = very good
• βρέχει πολύ = it rains a lot / heavily

Because Greek usually drops subject pronouns when they are not needed.

The ending -εις in βγαίνεις already tells you the subject is you singular.

So:

• βγαίνεις = you go out / you are going out
• μην βγαίνεις = don’t go out

You could add εσύ for emphasis, but normally you do not need it:

• Εσύ μην βγαίνεις έξω όταν βρέχει πολύ.
  This sounds more emphatic, like you specifically.

Yes. Greek word order is fairly flexible, and that version is perfectly natural:

• Όταν βρέχει πολύ, μην βγαίνεις έξω.

That version puts the when-clause first, which can make the condition sound more prominent.

Both are correct:

• Μην βγαίνεις έξω όταν βρέχει πολύ.
• Όταν βρέχει πολύ, μην βγαίνεις έξω.

The difference is mostly one of focus and style, not basic meaning.

The sentence is singular, because βγαίνεις is 2nd person singular.

To say it to more than one person, you would use:

• Μην βγαίνετε έξω όταν βρέχει πολύ.

So:

• βγαίνεις = you singular
• βγαίνετε = you plural

It can be any of those, depending on tone and context.

By itself, Μην βγαίνεις έξω όταν βρέχει πολύ often sounds like:

• practical advice
• a warning
• caring instruction

It is not automatically harsh. Greek negative commands with μην are very common and can sound completely normal and polite in everyday speech.

So the force comes less from the grammar itself and more from who says it, how they say it, and in what situation.