Learn Hardcore Greek: Όταν περνάς τον δρόμο, χρειάζεται προσοχή. - You need to pay attention when you cross the road.

Όταν is used for things that actually happen (real time) and often for repeated situations:
- Όταν περνάς τον δρόμο, χρειάζεται προσοχή.
  = When(ever) you cross the road, you need to be careful.
- Όταν βρέχει, μένω σπίτι.
  = When(ever) it rains, I stay home.
Αν means if and introduces a condition:
- Αν περνάς τον δρόμο χωρίς να κοιτάς, είναι επικίνδυνο.
  = If you cross the road without looking, it is dangerous.

Sometimes in English both are translated with when, but in Greek:

όταν = time
αν = condition

Where is the word you in Greek? Why don’t we see εσύ?

The you is contained in the verb ending -άς of περνάς.

Greek verb endings show the person, so you don’t need to say the subject pronoun unless you want to emphasize it:

(Εσύ) περνάς = you cross / you are crossing
(Εγώ) περνάω = I cross
(Αυτός/αυτή) περνά = he / she crosses

In this sentence, εσύ is omitted because it’s clear from περνάς that the subject is you (2nd person singular).

What is the verb περνάς? What form is it and what does it literally mean?

Περνάς comes from the verb περνάω / περνώ.

Person: 2nd person singular (you)
Tense: present
Mood: indicative
Voice: active

So περνάς = you pass / you are crossing.

Literal meanings of περνάω / περνώ:

to pass (by, through)
to cross (a street, a river, etc.)
to spend (time), e.g. περνάω καλά = I have a good time

In Όταν περνάς τον δρόμο, it means when you cross / when you are crossing the road.

Why is περνάς in the present tense if the sentence is giving general advice?

Greek often uses the present tense to talk about general truths, habits, and repeated situations. This is similar to the present simple in English:

Όταν περνάς τον δρόμο, χρειάζεται προσοχή.
= When(ever) you cross the road, you need to be careful.

It doesn’t refer to one specific time, but to any time you cross the road. So the present tense here is generic / habitual, not just “right now”.

Is there any difference between περνάω and περνώ?

Functionally, no meaningful difference in modern everyday Greek:

περνάω and περνώ are two accepted forms of the same verb.
Both mean to pass / to cross / to spend (time).

Typical present conjugation:

Εγώ περνάω / περνώ – I pass / I cross
Εσύ περνάς – You pass / you cross
Αυτός περνάει / περνά – He passes / he crosses

You will see both -άω and -ώ type verbs in Greek (γράφω–γράφω doesn’t change, but τηλεφωνάω–τηλεφωνώ, κτλ.). Both versions of περνάω / περνώ are correct.

Why do we say τον δρόμο and not just δρόμο? What does τον show?

Τον is the definite article (the) in the accusative singular masculine.

ο δρόμος = the road (subject form, nominative)
τον δρόμο = the road (object form, accusative)

Here τον δρόμο is the direct object of the verb περνάς:

Περνάς τον δρόμο. = You cross the road.

In Greek, the article is used much more often than in English, even for general ideas:

Περνάς τον δρόμο ≈ You cross the road (in general, not a specific road)
Leaving out the article (περνάς δρόμο) is unusual and sounds incomplete or unnatural in this context.

So τον tells us:

the noun is masculine singular
it is in the accusative (object) case
it is definite (like English the).

Why does δρόμος change to δρόμο?

This is a case change: nominative → accusative.

Δρόμος is a regular masculine noun:

ο δρόμος (nominative, subject) – the road
τον δρόμο (accusative, object) – the road

Examples:

Ο δρόμος είναι στενός. = The road is narrow. (subject: ο δρόμος)
Περνάς τον δρόμο. = You cross the road. (object: τον δρόμο)

The ending -ος → -ο is a normal pattern for many masculine nouns when they become direct objects.

What does χρειάζεται mean exactly, and why is there no it?

Χρειάζεται comes from χρειάζομαι, which means to need / to be needed.

In this sentence:

Χρειάζεται προσοχή. = Attention is needed / Care is needed.

Grammatically:

χρειάζεται is 3rd person singular
προσοχή is the subject (feminine nominative), placed after the verb.

So the structure is like:

[Is-needed] [attention].

In English we usually say:

You need to be careful.
Care is needed.
It is necessary to be careful.

Greek does not need a dummy subject like English it. The real subject is προσοχή.

What does προσοχή mean, and what kind of word is it?

Προσοχή is a feminine noun meaning:

attention
care
caution

It is related to the verb προσέχω = to pay attention / to be careful.

So:

χρειάζεται προσοχή = attention is necessary / you must be careful.

You also see προσοχή! on signs, meaning Caution! / Attention!

Why is the word order χρειάζεται προσοχή and not η προσοχή χρειάζεται?

Both are grammatically possible, but they feel slightly different:

Χρειάζεται προσοχή.
Very natural and common; verb-first. It sounds like a general statement:
Care is needed. / One must be careful.
Η προσοχή χρειάζεται.
Also grammatical, but less natural here; it sounds more like you are specifically talking about attention itself (e.g. in a list of things that are needed).

In Greek, putting the verb first is common in neutral statements, especially when:

you make a general observation
the subject is a general idea like προσοχή, υπομονή, etc.

So Χρειάζεται προσοχή is the most idiomatic word order for this meaning.

How would you say this in a more direct, personal way like “You must be careful when you cross the road.”?

Common, natural options:

Όταν περνάς τον δρόμο, πρέπει να προσέχεις.
= When you cross the road, you must be careful.
Όταν περνάς τον δρόμο, πρέπει να είσαι προσεκτικός.
(to a man)
= When you cross the road, you must be careful.
Όταν περνάς τον δρόμο, πρέπει να είσαι προσεκτική.
(to a woman)
More literal to the original:
- Όταν περνάς τον δρόμο, χρειάζεται να προσέχεις.

The original sentence χρειάζεται προσοχή is a bit more impersonal / general (care is needed), while πρέπει να προσέχεις talks directly to you.

Is περνάς informal? How would I say this politely to a stranger or to more than one person?

Περνάς is 2nd person singular, used for:

talking to one person informally (like English you to a friend)
or as a generic “you” (talking about people in general)

For polite or plural you, use 2nd person plural:

Όταν περνάτε τον δρόμο, χρειάζεται προσοχή.
= When you (formal or plural) cross the road, care is needed.

So:

περνάς → one person, informal
περνάτε → polite you or you (all)

How do you pronounce this sentence, especially ντ in τον δρόμο and the stress?

Standard pronunciation (roughly):

Όταν → [ˈotan] (stress on Ό)
περνάς → [perˈnas] (stress on νάς)
τον δρόμο → [ton ˈðromo]
- ντ is pronounced like nd or often close to d.
- δ in δρόμο is like English th in this.
- stress on δρό: ΔΡΌ-mo
χρειάζεται → [xriˈazete] (stress on ά)
προσοχή → [prosoˈçi] (stress on χή)
- χ is a voiceless kh sound (like German Bach).

Whole sentence:

Όταν περνάς τον δρόμο, χρειάζεται προσοχή.
→ [ˈotan perˈnas ton ˈðromo xriˈazete prosoˈçi]

Main stressed syllables: Ό–νάς–δρό–ά–χή.

Could we also use οδός instead of δρόμος? What’s the difference?

Δρόμος and οδός are both words for road / street, but:

δρόμος is the normal everyday word.
οδός is formal and used mainly:
- in street names:
  οδός Πατησίων, οδός Σταδίου
- in official or technical language.

Grammatically:

ο δρόμος (masc.) – nominative
τον δρόμο – accusative
η οδός (fem.) – nominative
την οδό – accusative

You could say:

Όταν περνάς την οδό, χρειάζεται προσοχή.

…but in everyday speech this sounds overly formal or strange.
For a general sentence like this, δρόμος is the natural choice:

Όταν περνάς τον δρόμο, χρειάζεται προσοχή.