Στην κατηφόρα πρέπει να προσέχεις, γιατί ο δρόμος γλιστράει όταν βρέχει.

Στην is the contracted form of σε την.

In Modern Greek, σε often combines with the definite article:

• σε + την → στην
• σε + το → στο
• σε + τον → στον

So στην κατηφόρα means on the downhill / when going downhill.

The basic dictionary form is η κατηφόρα. It is a feminine noun meaning downhill slope, descent, or sometimes more generally going downhill.

After σε, Greek uses the accusative case, so:

• nominative: η κατηφόρα
• accusative: την κατηφόρα

That is why you get στην κατηφόρα.

Literally, yes, it is something like in/on the downhill. But idiomatically, it means on a downhill stretch, when going downhill, or while descending.

Greek often uses prepositional phrases like this in ways that sound more natural in Greek than in literal English.

In Modern Greek, πρέπει is usually followed by να plus a finite verb.

So:

• πρέπει = it is necessary / one must / you must
• να προσέχεις = to be careful / that you be careful

Modern Greek does not normally use an infinitive the way English does. Instead of something like must be careful with an infinitive, Greek uses πρέπει να + verb.

Greek often uses the second person singular for general advice or instructions, just like English uses generic you.

So πρέπει να προσέχεις does not necessarily mean the speaker is addressing one specific person. It can mean:

• you have to be careful
• one must be careful
• you should be careful

It is a very natural way to give general advice.

This is about aspect.

• να προσέχεις uses the imperfective aspect and suggests something ongoing, repeated, or habitual.
• να προσέξεις uses the perfective aspect and sounds more like a single specific act: be careful!, make sure you watch out.

In this sentence, the meaning is general: whenever you are going downhill, you should be careful. That is why προσέχεις fits better.

The verb is προσέχω, which can mean:

• pay attention
• be careful
• watch out

In this sentence, be careful is the best natural meaning.

So πρέπει να προσέχεις = you have to be careful.

Yes, γιατί can mean both why? and because.

The role depends on the sentence:

• In a question: Γιατί άργησες; = Why were you late?
• In a statement: Άργησα γιατί είχε κίνηση. = I was late because there was traffic.

Here it introduces a reason, so it means because.

Greek uses the definite article much more often than English.

Ο δρόμος is the subject of γλιστράει, and the article is perfectly normal even in a general statement. Greek often says the road where English might choose the road, roads, or even no article depending on style.

So ο δρόμος γλιστράει is a natural Greek way to say that the road surface becomes slippery.

The verb γλιστράω / γλιστράει means slip, skid, or be slippery depending on context.

With ο δρόμος, the natural English meaning is the road is slippery.

So Greek uses a verb where English often prefers an adjective:

• ο δρόμος γλιστράει = the road is slippery
• compare with ο δρόμος είναι γλιστερός = the road is slippery

Both are possible, but γλιστράει gives a slightly more dynamic sense.

Yes. Many verbs in -άω have two common present-tense forms in Modern Greek.

For this verb, both of these are used:

• γλιστράει
• γλιστρά

They mean the same thing here. Γλιστράει is very common and completely standard.

Again, this is about aspect and type of meaning.

• όταν βρέχει = when it is raining / whenever it rains
  This is habitual or general.
• όταν βρέξει = when it rains / once it rains in a more specific future or one-time sense.

This sentence is giving a general truth: downhill roads are slippery whenever it rains. So όταν βρέχει is the natural choice.

Weather verbs in Greek are usually impersonal, just as in English.

• βρέχει = it rains / it is raining

The it is not a real thing; it is just the normal grammatical form. Greek does the same kind of thing here with the third person singular.

Greek word order is flexible. Putting Στην κατηφόρα first sets the scene immediately: as for downhill sections / when going downhill...

A more neutral order would also be possible:

• Πρέπει να προσέχεις στην κατηφόρα, γιατί ο δρόμος γλιστράει όταν βρέχει.

The original order sounds natural and slightly emphasizes the situation you should watch out for.

The comma separates the main statement from the reason clause:

• main statement: Στην κατηφόρα πρέπει να προσέχεις
• reason: γιατί ο δρόμος γλιστράει όταν βρέχει

This is very similar to English punctuation with because clauses when they are added as an explanation after the main clause.