Όταν οδηγώ, δεν απαντάω στο τηλέφωνο.

Όταν means when or whenever. It introduces a time clause:

• Όταν οδηγώ = when I drive
• In this kind of sentence, it often has a habitual sense, so whenever I drive is also a very natural understanding.

Because Greek present tense is often used for habits and general situations, not just for something happening right now.

So here:

• οδηγώ = I drive / I’m driving
• απαντάω = I answer / I’m answering

In this sentence, the overall meaning is habitual: Whenever I drive, I don’t answer the phone.

It can overlap with both in English, depending on context.

Greek does not separate I drive and I’m driving as strictly as English does. So Όταν οδηγώ can mean:

• when I drive
• when I’m driving
• while I’m driving
• whenever I drive

In this sentence, the most natural idea is a general rule or habit.

Because Greek usually drops the subject pronoun when it is already clear from the verb ending.

• οδηγώ already means I drive
• απαντάω already means I answer

So εγώ is unnecessary unless you want emphasis or contrast:

• Εγώ δεν απαντάω στο τηλέφωνο όταν οδηγώ.
  = I don’t answer the phone when I drive.

Δεν is the normal word for not in ordinary statements.

• απαντάω = I answer
• δεν απαντάω = I do not answer / I don’t answer

Greek usually puts δεν directly before the verb.

Because δεν is used to negate regular statements in the indicative.

μη(ν) is used in different environments, especially with:

• commands
• prohibitions
• subjunctive-style structures

For example:

• Δεν απαντάω στο τηλέφωνο. = I don’t answer the phone.
• Μην απαντάς στο τηλέφωνο. = Don’t answer the phone.

So in your sentence, δεν is the correct choice.

Yes. Απαντάω and απαντώ are two common forms of the same verb, meaning to answer or to reply.

In everyday Greek:

• απαντάω is very common
• απαντώ is also correct and often sounds a bit shorter or slightly more formal/neutral

So these are both possible:

• Δεν απαντάω στο τηλέφωνο.
• Δεν απαντώ στο τηλέφωνο.

Both are natural Greek.

Στο is a contraction of σε + το.

• σε = to / at / on / in depending on context
• το = the for a neuter singular noun
• τηλέφωνο is neuter singular

So:

• σε το τηλέφωνο becomes στο τηλέφωνο

This kind of contraction is very common in Greek:

• στο = σε + το
• στη(ν) = σε + τη(ν)
• στον = σε + τον
• στα = σε + τα

Because the verb απαντάω / απαντώ often takes σε in Greek for the person or thing being answered.

So Greek says, literally, something like I answer to the phone, but the natural English translation is simply I answer the phone.

This is just a normal Greek verb pattern that you learn together with the verb:

• απαντάω σε κάποιον = answer someone
• απαντάω σε μια ερώτηση = answer a question
• απαντάω στο τηλέφωνο = answer the phone

Because Όταν οδηγώ is a time clause placed before the main clause.

Greek commonly separates that opening clause with a comma:

• Όταν οδηγώ, δεν απαντάω στο τηλέφωνο.

You can also reverse the order:

• Δεν απαντάω στο τηλέφωνο όταν οδηγώ.

That is also correct, and in that order the comma is often left out.

Then Greek often changes the verb form after όταν.

A common way is:

• Όταν οδηγήσω, δεν θα απαντήσω στο τηλέφωνο.

This suggests a specific future case, not a general habit.

So the contrast is roughly:

• Όταν οδηγώ, δεν απαντάω στο τηλέφωνο.
  = Whenever I drive, I don’t answer the phone.

• Όταν οδηγήσω, δεν θα απαντήσω στο τηλέφωνο.
  = When I drive / when I get behind the wheel later, I won’t answer the phone.

This is a more advanced point, but it is very useful because Greek often marks the difference between habitual and single future actions this way.