Learn Hardcore Greek: Το πρωί δεν βιάζομαι, γιατί συνήθως δεν έχει πολύ κίνηση στον δρόμο, μόνο περίπου δέκα αυτοκίνητα. - In the morning I am not in a hurry, because usually there is not much traffic on the road, only about ten cars.

πρωί is neuter, so it would be στο πρωί, not στην (which is for feminine nouns).
Even στο πρωί is not what people say for this meaning; the idiomatic time expression is just το πρωί.

So: Το πρωί δεν βιάζομαι = “(In) the morning I am not in a hurry.”

What exactly does δεν βιάζομαι mean, and why is it middle/reflexive instead of using είμαι (“to be”)?

Βιάζομαι is a middle/reflexive verb and it means “I am in a hurry / I hurry.” It is not built with είμαι.

βιάζω = I hurry someone / I pressure someone (active, transitive, less common in everyday speech)
βιάζομαι = I’m in a hurry / I rush (middle/reflexive, very common)

So:

Δεν βιάζομαι = “I’m not in a hurry” or “I’m not rushing.”
A possible alternative is δεν είμαι βιαστικός/η, but this sounds more like “I’m not a hurried type (by nature)” rather than “right now / generally in the mornings I’m not in a hurry.”

Here, δεν βιάζομαι is the natural choice for the situation.

Does γιατί here mean “why” or “because”, and is the comma before it necessary?

Γιατί can mean both:

“why?” – Γιατί δεν βιάζεσαι; = “Why aren’t you in a hurry?”
“because” – Δεν βιάζομαι, γιατί δεν έχει κίνηση. = “I’m not in a hurry, because there isn’t much traffic.”

In your sentence it clearly means “because”:

...δεν βιάζομαι, γιατί συνήθως δεν έχει πολύ κίνηση...

The comma is normal and helpful: it separates the main clause (Δεν βιάζομαι) from the reason clause (γιατί...). In writing, you almost always put that comma there.

What does συνήθως mean and where can it go in the sentence?

Συνήθως means “usually”.

In your sentence:

γιατί συνήθως δεν έχει πολύ κίνηση στον δρόμο...

This word order is very natural: “because usually there isn’t much traffic...”.

You can move συνήθως around a bit:

γιατί δεν έχει συνήθως πολύ κίνηση στον δρόμο
γιατί δεν έχει πολύ κίνηση συνήθως στον δρόμο

All are grammatical, but the most neutral/common here is the original: γιατί συνήθως δεν έχει πολύ κίνηση στον δρόμο.

Notice that συνήθως doesn’t change form; it’s an adverb.

Why is it δεν έχει πολύ κίνηση and not something like δεν υπάρχει πολύ κίνηση for “there isn’t much traffic”?

Greek often uses έχω impersonally, where English uses “there is/are”:

Έχει πολύ κίνηση. = “There is a lot of traffic.”
Δεν έχει πολύ κίνηση. = “There isn’t much traffic.”

This έχει has no explicit subject (no “it”). It’s just “there is/are”.

You can say:

Δεν υπάρχει πολλή κίνηση.

This is also correct, but (δεν) έχει κίνηση is more colloquial and common in everyday speech when talking about traffic, weather, noise, people, etc.:

Έχει ζέστη – It’s hot.
Έχει κόσμο – There are (a lot of) people.
Έχει φασαρία – It’s noisy.

Why is it πολύ κίνηση and not πολλή κίνηση? Aren’t both possible?

With κίνηση (feminine noun), the fully agreeing adjective form would be:

πολλή κίνηση = a lot of traffic / lots of movement

However, in everyday modern Greek, πολύ is very commonly used in front of feminine (and masculine) nouns as well, especially in fixed expressions:

πολύ κίνηση
πολύ δουλειά
πολύ φασαρία

Grammatically:

πολλή is the strictly correct feminine form of the adjective “much/many”.
πολύ works both as an adjective (neuter form) and as an adverb (“a lot / very”).

So:

δεν έχει πολύ κίνηση is fully natural colloquial Greek.
δεν έχει πολλή κίνηση is more “school-correct”, but also understood and acceptable.

Learners will encounter πολύ κίνηση all the time in real speech.

What exactly does κίνηση mean here? Can it mean just “movement” too?

Yes, κίνηση literally means “movement, motion”, and by extension, very commonly:

traffic (on the road)

So:

Έχει πολλή κίνηση στον δρόμο. = There’s a lot of traffic on the road.
Κάνω λίγη κίνηση = I move a bit / I do some movement (e.g. exercise).

In your sentence it clearly means traffic: cars on the road. Another word is κυκλοφορία, but κίνηση is more colloquial and widely used in this sense.

Why is it στον δρόμο and sometimes I see στο δρόμο or στο δρομο written?

The “full” form is:

σε + τον δρόμο → στον δρόμο = “on the road”

In fast everyday speech, many speakers pronounce this more like στο δρόμο, almost dropping the ν of στον. In writing:

στον δρόμο = fully standard spelling
στο δρόμο = also very common, reflecting the way people actually speak

You might also see the noun written as δρόμο or δρόμο with the accent on the first syllable only (the accent is fixed in the nominative: δρόμος, and stays in the same place in other cases: του δρόμου, στον δρόμο).

So the important parts for you:

Preposition + article: σε
- τον → στον
Noun in accusative: ο δρόμος → στον δρόμο

Why do we need the article τον in στον δρόμο? Could we just say σε δρόμο?

In Greek, you very often use the definite article even for general or generic things where English has no article.

στον δρόμο literally “on the road”, but often means simply “on the road, out on the street” in general.

Saying σε δρόμο is also possible, but it typically means “on a (certain/unspecified) road” rather than the generic “on the road”:

Υπάρχει κίνηση σε δρόμο της Αθήνας. = There is traffic on a road in Athens.

So for the everyday idea “on the road (when I drive to work etc.)”, στον δρόμο is the normal expression.

What is the function of μόνο and περίπου together in μόνο περίπου δέκα αυτοκίνητα? Could I change the order?

μόνο = only
περίπου = about / approximately

Together:

μόνο περίπου δέκα αυτοκίνητα = “only about ten cars”

This means both “not many” (μόνο) and “it’s an approximate number” (περίπου).

Order:

μόνο περίπου δέκα αυτοκίνητα – very natural, perhaps the most common.
περίπου μόνο δέκα αυτοκίνητα – also possible, but sounds a bit heavier/emphatic.

You can also move μόνο to the end:

περίπου δέκα αυτοκίνητα μόνο = about ten cars only

All are understandable. The original order is what you’ll hear most in casual speech.

Why is there no verb in μόνο περίπου δέκα αυτοκίνητα? Is that a complete sentence in Greek?

At the end of the original sentence:

..., μόνο περίπου δέκα αυτοκίνητα.

Greek, like English in informal speech, often drops a repeated, obvious verb in such cases. The full version would be:

..., μόνο περίπου δέκα αυτοκίνητα (έχει).
“..., only about ten cars (there are).”

The verb έχει (“there are / there is”) is understood from the previous clause δεν έχει πολύ κίνηση and doesn’t need to be repeated.

So it’s not a full standalone sentence; it’s an ellipsis inside a longer sentence, which is very natural in speech and informal writing.

Why is it δρόμο here and not δρόμος?

Δρόμος is a masculine noun. Here it appears after the preposition σε:

σε + τον δρόμο → στον δρόμο

Prepositions like σε normally require the accusative case. For masculine nouns:

Nominative: ο δρόμος (subject)
Accusative: τον δρόμο (after prepositions, direct object)

So:

Ο δρόμος είναι άδειος. – The road is empty. (subject, nominative)
Δεν έχει κίνηση στον δρόμο. – There’s no traffic on the road. (after σε, accusative)

Why is the present tense (δεν βιάζομαι, δεν έχει) used for a general habit rather than a separate form like English “I don’t usually hurry in the morning”?

Modern Greek uses the present simple for both:

actions happening right now, and
general truths / repeated habits.

So:

Δεν βιάζομαι. – I’m not in a hurry (now / in general).
Συνήθως δεν βιάζομαι το πρωί. – I usually don’t hurry in the morning.

There is no separate “present simple vs present continuous” form like in English. The distinction (habit vs now) is understood from context and from time adverbs like συνήθως, κάθε μέρα, το πρωί, etc.