Μην ξεχνάς το ερωτηματικό όταν γράφεις μια ερώτηση.

Because Greek makes negative commands with μη(ν) + a verb form, not with a separate word like English don't.

So:

• Μην ξεχνάς = Don’t forget
• not Δεν ξεχνάς, because δεν is used for plain negation in statements, not commands.

A useful rule:

• δεν = negates statements
• μην = negates commands, wishes, and some subjunctive-type constructions

Ξεχνάς is:

• 2nd person singular
• present tense
• active voice
• from ξεχνάω / ξεχνώ = to forget

So it literally means you forget / you are forgetting, but after μην it functions as don’t forget.

Because it is 2nd person singular, the speaker is talking to one person informally.

Both are possible, but they are not exactly the same.

• Μην ξεχνάς uses the imperfective idea and sounds like a general reminder / repeated habit.
• Μην ξεχάσεις uses the aorist/perfective idea and often sounds like don’t forget on that one occasion.

In this sentence, there is a general situation: when you write a question. So Μην ξεχνάς fits the idea of a standing rule very well.

Very roughly:

• Μην ξεχνάς = Don’t go forgetting / Remember, as a rule
• Μην ξεχάσεις = Don’t forget this time

Το ερωτηματικό means the question mark.

Grammar-wise:

• το = the for a neuter singular noun
• ερωτηματικό = question mark

So το ερωτηματικό is the direct object of ξεχνάς.

Also, an important Greek punctuation fact:

• the Greek question mark is written as ;
• not as ?

So ερωτηματικό refers to that Greek punctuation sign.

Greek uses articles more often than English does. In this sentence, το ερωτηματικό sounds natural because it refers to the punctuation mark that should be there.

English often says:

• Don’t forget the question mark

Greek does the same here:

• Μην ξεχνάς το ερωτηματικό

Without the article, ερωτηματικό would sound less natural in this context.

Because ερώτηση is a feminine singular noun, and here it means a question.

So:

• μια = a / one for feminine singular nouns
• ερώτηση = question

Together:

• μια ερώτηση = a question

It is indefinite because the sentence means any question you write, not one specific question.

Good question. Μια ερώτηση is the object of γράφεις, but in this noun the nominative and accusative singular forms are the same.

So:

• nominative: η ερώτηση
• accusative: την ερώτηση

The article changes clearly, but the noun itself stays ερώτηση.

That is why after μια you still see ερώτηση.

Γράφεις is:

• 2nd person singular
• present tense
• from γράφω = to write

It means you write / you are writing.

Here it matches ξεχνάς because the speaker is still talking to the same person:

• when you write a question

Because in Modern Greek, after words like όταν (when), Greek normally does not use θα the way English uses will.

English says:

• when you write not usually
• when you will write

Greek works similarly, but even more strongly:

• όταν γράφεις
• not normally όταν θα γράφεις in this kind of sentence

So the future or general-time meaning comes from the whole sentence and context, not from θα here.

Both can exist, but the difference is aspect.

• όταν γράφεις = when(ever) you write; general, repeated, habitual
• όταν γράψεις = more like when you write/once you have written in a specific future instance

Because this sentence gives a general instruction, όταν γράφεις is very natural.

So the sentence is closer to:

• Whenever you write a question, don’t forget the question mark.

It is informal singular, because of:

• ξεχνάς
• γράφεις

These are forms used for one person in the familiar you.

If you wanted plural or formal singular, you would say:

• Μην ξεχνάτε το ερωτηματικό όταν γράφετε μια ερώτηση.

So:

• -άς / -εις = singular informal
• -άτε / -ετε = plural or formal

No, Greek word order is fairly flexible.

The given sentence is:

• Μην ξεχνάς το ερωτηματικό όταν γράφεις μια ερώτηση.

You could also say:

• Όταν γράφεις μια ερώτηση, μην ξεχνάς το ερωτηματικό.

Both are natural. The difference is mostly emphasis:

• starting with Μην ξεχνάς... emphasizes the command first
• starting with Όταν γράφεις... sets the situation first

Because Greek often does not use a comma before a short subordinate clause when it comes after the main clause.

So this is normal:

• Μην ξεχνάς το ερωτηματικό όταν γράφεις μια ερώτηση.

If the όταν clause comes first, a comma is much more likely:

• Όταν γράφεις μια ερώτηση, μην ξεχνάς το ερωτηματικό.

So the punctuation is behaving quite naturally in Greek.