Στην κατηφόρα κρατάω χαμηλή ταχύτητα, γιατί ο δρόμος γλιστράει.

Because it is the usual contracted form of σε την.

• σε = in / on / at / to
• την = the for a feminine noun in the accusative

So:

• σε την κατηφόρα → στην κατηφόρα

This kind of contraction is extremely common in Modern Greek:

• σε τον → στον
• σε το → στο
• σε τη(ν) → στη(ν)

In this sentence, στην is the normal form.

It is in the accusative.

After the preposition σε, Modern Greek normally uses the accusative, whether the meaning is:

• location: in / on / at
• direction: to

So in στην κατηφόρα, the article την shows the accusative.

A useful thing to notice: with many feminine nouns ending in -α, the noun itself looks the same in nominative and accusative:

• nominative: η κατηφόρα
• accusative: την κατηφόρα

So the article is what makes the case especially clear here.

Κατηφόρα is a noun meaning downhill slope, descent, or downhill stretch of road.

So στην κατηφόρα means something like:

• on the downhill
• when going downhill
• on a downward slope

The opposite word is ανηφόρα, meaning uphill.

Yes, κρατώ is also correct.

The verb κρατάω / κρατώ means to hold, to keep, or to maintain.
In this sentence, it means I keep / maintain a low speed.

So:

• κρατάω χαμηλή ταχύτητα = I keep a low speed
• κρατώ χαμηλή ταχύτητα = same meaning

In everyday speech, κρατάω is very common.
Κρατώ can sound a bit shorter, sometimes slightly more formal or stylistically tighter, but both are standard.

Because Greek can express this idea with a noun phrase: low speed.

Here:

• χαμηλή = low
• ταχύτητα = speed

So the structure is literally I keep low speed.

That is natural Greek. But Greek also has other ways to express a similar idea, for example:

• κρατάω χαμηλά την ταχύτητα = I keep the speed low
• οδηγώ αργά = I drive slowly

These are close in meaning, but not exactly identical in nuance:

• χαμηλή ταχύτητα focuses on the speed level
• αργά focuses on the manner of driving

Because it agrees with ταχύτητα, which is a feminine noun.

Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

Here:

• ταχύτητα is feminine singular
• it is in the accusative
• so the adjective must match it: χαμηλή

That is why we get:

• χαμηλή ταχύτητα

Because the phrase is indefinite and general.

Greek often omits the article when the meaning is like:

• a low speed
• low speed in general

So:

• κρατάω χαμηλή ταχύτητα = I keep a low speed

If you added the article:

• κρατάω τη χαμηλή ταχύτητα

it would sound more like I keep the low speed, referring to a specific speed already known from the context. That is not the idea here.

Yes, επειδή could also work.

In this sentence, γιατί means because:

• ..., γιατί ο δρόμος γλιστράει.
• ..., because the road is slippery.

You could also say:

• ..., επειδή ο δρόμος γλιστράει.

The difference is mostly about style and tone:

• γιατί is very common and natural in everyday speech
• επειδή can sound a bit more formal, explanatory, or careful

Also note that γιατί can mean both:

• because
• why

The punctuation and sentence type tell you which meaning it has.

Because Greek uses the definite article much more often than English.

Here ο δρόμος literally means the road, but in context it refers naturally to the road you are on or the road surface in general. Greek usually prefers the article in cases like this.

So even where English might sometimes say something more general, Greek often says:

• ο δρόμος
• το αυτοκίνητο
• η βροχή

Using the article here is completely normal.

Yes. Γλιστράει and γλιστρά are both possible.

This is the 3rd person singular present form of the verb γλιστράω, meaning to slip.

So:

• ο δρόμος γλιστράει
• ο δρόμος γλιστρά

Both mean the same thing here.

In this sentence, with road as the subject, the meaning is not that the road is physically moving. It means:

• the road is slippery
• literally, the road slips

That kind of literal Greek phrasing is very common.

Because the sentence expresses a general/habitual situation or a current real situation.

• κρατάω = I keep / I maintain
• γλιστράει = it is slippery / it slips

The Greek present can cover both:

• what someone generally does
• what is happening now

So the sentence can mean either:

• This is what I do when going downhill or
• I am doing this now, because the road is slippery now

The exact interpretation comes from context.

Greek word order is fairly flexible.

The version you have is very natural:

• Στην κατηφόρα κρατάω χαμηλή ταχύτητα, γιατί ο δρόμος γλιστράει.

It starts with the setting:

• Στην κατηφόρα = On the downhill

Then it gives the action:

• κρατάω χαμηλή ταχύτητα

Then the reason:

• γιατί ο δρόμος γλιστράει

Other orders are possible, but they may shift emphasis. For example:

• Κρατάω χαμηλή ταχύτητα στην κατηφόρα, γιατί ο δρόμος γλιστράει.
• Γιατί ο δρόμος γλιστράει, κρατάω χαμηλή ταχύτητα στην κατηφόρα.

The original order sounds very natural and clear.

Because γιατί ο δρόμος γλιστράει is a clause giving the reason for the main clause.

Greek often uses a comma before explanatory or causal clauses introduced by γιατί, especially in clear written style.

So the comma helps separate:

• the main statement: Στην κατηφόρα κρατάω χαμηλή ταχύτητα
• the reason: γιατί ο δρόμος γλιστράει

In informal writing, punctuation can sometimes be looser, but the comma here is normal and appropriate.