Μην αφήνεις την καφετιέρα ανοιχτή όταν φεύγεις από το σπίτι.

Because Μην is used for negative commands and prohibitions.

• Δεν = negates a statement
• Μην = negates an instruction like don’t ...

So:

• Δεν αφήνεις... would mean something like you are not leaving...
• Μην αφήνεις... means don’t leave...

Also, the final -ν in μην is kept here because the next word, αφήνεις, begins with a vowel.

This is an aspect choice.

• Μην αφήνεις uses the imperfective form
• Μην αφήσεις uses the perfective form

In a sentence like this, Μην αφήνεις sounds like a general instruction or habitual warning: whenever you leave, don’t leave the coffee maker on/open.

By contrast, Μην αφήσεις would sound more like a warning about one specific occasion.

So:

• Μην αφήνεις... = Don’t leave it on/open (as a general rule)
• Μην αφήσεις... = Don’t leave it on/open (this one time / in this instance)

Yes. Αφήνεις is second person singular, so it is addressed to one person, informally.

If you were speaking to:

• more than one person, or
• one person formally

you would use αφήνετε instead:

Μην αφήνετε την καφετιέρα ανοιχτή όταν φεύγετε από το σπίτι.

Την is the feminine singular accusative definite article, meaning the.

Here, την καφετιέρα is the direct object of αφήνεις.

So:

• η καφετιέρα = the coffee maker as subject
• την καφετιέρα = the coffee maker as object

Greek uses articles very often, and in this sentence it refers to a specific, understood coffee maker.

Because ανοιχτή must agree with καφετιέρα.

Καφετιέρα is:

• feminine
• singular
• here in the accusative

So the adjective also appears in the matching form:

• masculine: ανοιχτός
• feminine: ανοιχτή
• neuter: ανοιχτό

This adjective is functioning as an object complement: it describes the state in which the coffee maker is being left.

So αφήνεις την καφετιέρα ανοιχτή literally means something like you leave the coffee maker open/on.

Literally, ανοιχτή means open. But with appliances, Greek often uses ανοιχτός / ανοιχτή / ανοιχτό to mean on, switched on, or left running.

So in this sentence, the natural meaning is:

• don’t leave the coffee maker on

Even though the adjective itself is the ordinary word for open.

Again, this is an aspect difference.

• όταν φεύγεις = when you leave / whenever you leave as a repeated or habitual action
• όταν φύγεις = when you leave in a more single, completed, future-oriented sense

In this sentence, όταν φεύγεις fits the idea of a regular instruction:

Whenever you leave the house, don’t leave the coffee maker on.

So the whole sentence has a general, routine feel.

Καφετιέρα usually means coffee maker, coffee machine, or sometimes coffee pot, depending on context.

In this sentence, it clearly refers to the appliance you might accidentally leave on.

So a learner should understand it here as coffee maker.

Greek often uses the definite article in places where English does not.

So:

• από το σπίτι literally = from the house
• but in normal English, it often translates naturally as from home or from the house, depending on context

Also, σπίτι can mean both house and home. Greek does not always separate those two ideas as sharply as English does.

Yes. Σπίτι is very commonly used for both.

Depending on context, το σπίτι may mean:

• the house
• the home
• home

So φεύγεις από το σπίτι can be understood as either:

• you leave the house
• you leave home

English chooses the version that sounds most natural in context.

Because after verbs like αφήνω, Greek very naturally uses:

verb + object + adjective

to show the state in which something is left.

So:

• αφήνω την πόρτα ανοιχτή = I leave the door open
• αφήνω το φως αναμμένο = I leave the light on
• αφήνω την καφετιέρα ανοιχτή = I leave the coffee maker on/open

Putting the adjective before the noun would not be the normal pattern here. The post-noun adjective works best because it describes the resulting state of the object.

It would be:

Μην αφήνετε την καφετιέρα ανοιχτή όταν φεύγετε από το σπίτι.

The only changes are:

• αφήνεις → αφήνετε
• φεύγεις → φεύγετε

Everything else stays the same.