Learn Hardcore Greek: Μην βάλεις το τηλέφωνο μέσα σε νερό.

Negative commands are made with μη(ν) + subjunctive. Here the aorist subjunctive 2nd person singular is βάλεις.
βάζεις is present (used for habits/ongoing actions; with μην it would ban ongoing/habitual putting).
βάλε is the positive imperative (Put!), which cannot be used with μην.

What’s the difference between Μην βάλεις and Μην βάζεις?

Aspect:

Μην βάλεις (aorist) = Don’t do it this one time / don’t perform that single action.
Μην βάζεις (present) = Don’t be putting it (now), or don’t do that as a habit.

Can I also say Να μην βάλεις το τηλέφωνο...?

Yes. Να μην + subjunctive also expresses prohibition/advice. It’s usually a bit softer or more advisory than the bare Μην βάλεις, which can sound more direct/urgent.

Should it be Μη βάλεις or Μην βάλεις?

Both are seen. Traditional “movable -ν” guidance: use μην before a vowel and before κ, π, τ, ξ, ψ (and clusters like μπ, ντ, γκ, τσ, τζ); use μη elsewhere. Since βάλεις starts with β, the strict rule gives Μη βάλεις. In modern usage many people just keep μην almost everywhere, so Μην βάλεις is very common and acceptable.

Why σε νερό and not στο νερό?

σε νερό = in water (any water, non-specific; mass-noun feel).
στο νερό (= σε + το νερό) = in the water (a specific body/source of water). Both are grammatically fine; choose based on definiteness.

What does μέσα add? Could I omit it?

μέσα (σε) emphasizes inside/into. Without it, σε already means in/into; the verb of motion supplies the “into” idea.

Μην βάλεις το τηλέφωνο στο νερό. (Don’t put the phone in/into the water.)
Μην βάλεις το τηλέφωνο μέσα στο νερό. (…inside the water, a bit stronger.)

What cases are used for το τηλέφωνο and νερό?

το τηλέφωνο is the direct object in the accusative.
σε governs the accusative, so (σε) νερό is also accusative. Mass nouns like νερό are often article-less when indefinite.

If I replace the noun with a pronoun, where does it go?

Object pronouns go before the verb (and after μην/να/θα):

Μην το βάλεις (μέσα) στο νερό. Not: ✗ Μην βάλεις το στο νερό.

How do I say “your phone” here?

Μην βάλεις το τηλέφωνό σου (στο/μέσα σε) νερό. Note the extra accent on τηλέφωνό before the enclitic σου (standard with proparoxytone nouns + enclitics). You can also say το κινητό σου for “your mobile.”

Is τηλέφωνο the usual word, or would Greeks say κινητό?

In everyday speech, το κινητό (mobile/cell) is very common. το τηλέφωνο is generic (any telephone). Both are fine; pick what you mean:

Μην βάλεις το κινητό (σου) στο νερό.

Why does βάζω become βάλεις?

It’s an irregular aorist stem. Key forms:

Present: βάζω
Aorist indicative: έβαλα
Subjunctive: να βάλω, να βάλεις, …
Positive imperative: βάλε / βάλτε Negative commands use μη(ν) + subjunctive: Μην βάλεις…

Can I move parts around, like Μην βάλεις μέσα σε νερό το τηλέφωνο?

Yes, Greek word order is flexible for emphasis. That variant is possible but puts focus on the location. The neutral, most common order here is the original: verb + object + prepositional phrase.