Μην ρίχνεις πολύ νερό στη γλάστρα, γιατί το φυτό μπορεί να χαλάσει.

In Modern Greek, μη(ν) is used to negate commands, instructions, wishes, and forms with να.

So:

• Μην ρίχνεις... = Don’t pour / Don’t put...
• Δεν ρίχνεις... would mean you don’t pour..., which is just a statement, not a command.

So because this sentence is giving advice or a warning, Μην is the correct choice.

Yes, this is one of the common things that confuses learners.

In negative commands, Greek does not usually use the ordinary imperative form. Instead, it uses:

• μη(ν) + subjunctive-type form

So:

• Μην ρίχνεις = Don’t pour / Don’t put
• not an imperative ending like a positive command such as Ρίξε!

That is why it looks like the regular you form. In Modern Greek, the form after μην is often identical in appearance to the present tense form.

It is addressed to one person, informally.

ρίχνεις is 2nd person singular: you pour / you are pouring

So the sentence is talking to one person, like:

• Don’t pour too much water into the flowerpot...

If you wanted to say it to more than one person, or politely to one person, you would say:

• Μην ρίχνετε πολύ νερό στη γλάστρα...

Yes, ρίχνω very often means throw, drop, or cast, but it is also used much more broadly than English throw.

Depending on context, it can mean:

• throw
• drop
• pour
• put in
• add

With water, ρίχνω νερό is a very natural Greek way to say:

• pour water
• put water in
• water something, depending on context

So here Μην ρίχνεις πολύ νερό is idiomatic Greek, even if a word-for-word translation sounds like Don’t throw much water.

Because νερό is being treated as an uncountable noun, like water in English.

So Greek uses:

• πολύ νερό = a lot of water / too much water

Compare:

• πολύ νερό = much water
• πολλά ποτήρια = many glasses

So πολύ here agrees with the idea of quantity, not with a plural count noun.

στη is the common contracted form of:

• σε + τη

So:

• στη γλάστρα = in the flowerpot / into the flowerpot

This contraction is extremely common in Greek:

• στο = σε + το
• στη = σε + τη
• στον = σε + τον

You should think of στη as a normal everyday form, not as something unusual.

Because γλάστρα is the object of the preposition σε.

In Modern Greek, σε is normally followed by the accusative case.
For γλάστρα, the accusative singular looks the same as the nominative:

• η γλάστρα = the flowerpot
• τη γλάστρα = the flowerpot
• στη γλάστρα = in/into the flowerpot

So even though the form looks unchanged, grammatically it is accusative after σε.

Greek uses the definite article more often than English, and in this sentence το φυτό sounds completely natural.

Here it most likely refers to a specific plant:

• the plant in that pot
• this plant you are taking care of

So:

• το φυτό = the plant

You could sometimes omit articles in special contexts, but here the article is the normal choice.

Also, φυτό is a neuter noun, so its article is το.

It can mean both, depending on how it is used.

In this sentence:

• γιατί = because

So:

• ..., γιατί το φυτό μπορεί να χαλάσει.
• ..., because the plant may get damaged.

But in a question:

• Γιατί; = Why?

This is very common in Greek, so learners quickly get used to reading it from context.

This is a very common Greek structure:

• μπορεί να + verb

It means:

• can
• may
• might

So:

• μπορεί να χαλάσει = it may get damaged / it might go bad

Breakdown:

• μπορεί = it can / it may
• να introduces the following verb
• χαλάσει is the verb form meaning get spoiled / get damaged / go bad

So the whole clause means that overwatering creates a possible bad result.

This is about aspect, which is very important in Greek.

• να χαλάσει uses the aorist form
• να χαλάει would use the present/imperfective form

Here, the aorist is natural because it refers to the result as a single possible event:

• the plant may end up damaged
• the plant may go bad

If you used να χαλάει, it would sound more like an ongoing or repeated process.

So in this warning, μπορεί να χαλάσει is the normal choice.

Right—χαλάω / χαλώ is a broad verb.

Depending on context, it can mean:

• break
• spoil
• ruin
• go bad
• get damaged

With a plant, το φυτό μπορεί να χαλάσει means something like:

• the plant may be damaged
• the plant may go bad
• the plant may be ruined

In natural English, you might even translate the whole idea as:

• the plant might suffer
• the plant could be harmed by too much water

Yes. This is another aspect difference.

• Μην ρίχνεις = Don’t keep pouring / Don’t pour (as a general habit or ongoing action)
• Μην ρίξεις = Don’t pour (this one time / as a single action)

In practice, the English translation may be the same, but Greek feels the difference.

In your sentence, Μην ρίχνεις πολύ νερό... sounds like general advice:

• Don’t put too much water in the pot
• meaning as a rule

If you were stopping someone from doing one specific act right now, Μην ρίξεις... could also be possible depending on context.