Όταν έχει παγετό, η ταχύτητα πρέπει να είναι μικρή.

Yes, literally it looks like it has frost, but in Modern Greek έχει is often used impersonally to mean there is / there are, especially in weather and environmental expressions.

So:

• έχει παγετό = there is frost / it is frosty
• compare:
  • έχει ζέστη = it is hot
  • έχει κρύο = it is cold
  • έχει αέρα = it is windy

So this is a very natural Greek way to talk about conditions.

Because after έχει, the noun works as the direct object, so it appears in the accusative case.

The dictionary form is:

• ο παγετός = frost

But in this sentence:

• έχει παγετό

we use the accusative singular:

• τον παγετό / παγετό

The article is omitted here, which is also normal in this kind of expression.

Greek often leaves out the article in fixed weather or condition expressions like this.

So:

• έχει παγετό sounds natural
• έχει τον παγετό would not work here

The sentence is talking about frost as a general condition, not about a specific frost already identified.

Here Όταν can be understood as when in a general sense, very close to whenever.

In this sentence, it does not mean one single future moment only. It means something like:

• whenever there is frost
• when there is frost

So the sentence expresses a general rule.

Because that first part is a subordinate clause:

• Όταν έχει παγετό = when there is frost

Greek commonly separates this kind of introductory clause from the main clause with a comma:

• Όταν έχει παγετό, η ταχύτητα πρέπει να είναι μικρή.

This is similar to English writing:

• When there is frost, speed must be low.

Greek uses the definite article much more often than English.

So η ταχύτητα here means:

• the speed

But in context it really refers to speed in a general or relevant sense, so English may translate it simply as speed.

This is very common in Greek. A noun often needs an article where English would not use one.

This is a very important Greek pattern:

• πρέπει = it is necessary / one must / must
• να
  • verb = introduces the dependent verb
• είναι = be

So:

• πρέπει να είναι = must be / has to be

A useful pattern to remember is:

• πρέπει να + verb

Examples:

• Πρέπει να φύγω. = I must leave.
• Πρέπει να προσέχεις. = You must be careful.
• Η ταχύτητα πρέπει να είναι μικρή. = The speed must be low/small.

Because the subject is η ταχύτητα, which is third person singular.

So the verb είμαι appears in the third person singular form:

• είμαι = I am
• είσαι = you are
• είναι = he/she/it is

Since η ταχύτητα = the speed, Greek uses:

• η ταχύτητα πρέπει να είναι μικρή

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

In Greek, adjectives must agree with the noun in:

• gender
• number
• case

So:

• η ταχύτητα = feminine singular
• therefore μικρή = feminine singular

Compare:

• ο δρόμος είναι μικρός = the road is small
• η ταχύτητα είναι μικρή = the speed is small/low
• το αυτοκίνητο είναι μικρό = the car is small

That is a very natural question. In English, we often say low speed, while Greek can use μικρή ταχύτητα very naturally.

So:

• μικρή ταχύτητα = low speed / slow speed
• χαμηλή ταχύτητα is also possible in many contexts

But μικρή is very common and idiomatic here. Greek often uses small where English prefers low.

Yes. Greek word order is more flexible than English.

The given sentence is completely natural:

• Όταν έχει παγετό, η ταχύτητα πρέπει να είναι μικρή.

But you could also hear variations like:

• Η ταχύτητα πρέπει να είναι μικρή όταν έχει παγετό.
• Όταν έχει παγετό, πρέπει η ταχύτητα να είναι μικρή.

The original version is clear and neutral. Putting Όταν έχει παγετό first emphasizes the condition.

Normally it expresses a general rule.

So it means something like:

• Whenever there is frost, speed should be low.

If Greek wanted to refer to one very specific situation, context would usually make that clear. On its own, this sentence sounds like advice, instruction, or a road-safety rule.

A simple pronunciation guide is:

• Όταν = O-tan
• έχει = E-hi
• παγετό = pa-ye-TO
• η ταχύτητα = i ta-HI-ti-ta
• πρέπει = PRE-pi
• να είναι = na E-ne
• μικρή = mi-KRI

Very roughly as a whole:

• O-tan E-hi pa-ye-TO, i ta-HI-ti-ta PRE-pi na E-ne mi-KRI.

A few useful notes:

• γ before ε in παγετό sounds like a soft y sound
• χ in έχει is a throaty sound, not like English h
• the written accent marks show which syllable is stressed

In context, yes, depending on tone.

Strictly speaking, πρέπει often corresponds to:

• must
• have to
• it is necessary

But in safety instructions or practical advice, English may sometimes prefer should for a more natural style.

So depending on context, the sentence could be rendered as:

• When there is frost, speed must be low.
• When there is frost, speed should be low.

The Greek itself is fairly strong and expresses necessity.