En la nebulo veturiloj moviĝas pli malrapide.

En la nebulo literally means in the fog.

It uses nebulo without -n because this phrase shows a location or situation, not movement into something.

• en la nebulo = in the fog
• en la nebulon = into the fog

So in this sentence, the vehicles are already moving while in foggy conditions. They are not being described as entering the fog.

La is the definite article, meaning the.

Here, en la nebulo means in the fog, referring to the fog as the relevant condition or environment. That sounds natural in Esperanto.

You may also sometimes see something like en nebulo, which can feel more general, like in fog or in foggy conditions, but en la nebulo is very normal and clear.

Veturiloj means vehicles.

It breaks down like this:

• vetur- = travel / transport-related root
• -il- = tool or instrument
• -o = noun ending
• -j = plural ending

So:

• veturilo = a vehicle
• veturiloj = vehicles

The suffix -il- often makes a word for a tool or device. A vehicle is, in a sense, a means or instrument of transport.

Because movas is usually transitive: it means moves something.

• La vento movas la foliojn = The wind moves the leaves.

But in this sentence, the vehicles themselves are moving. Nothing is being acted on as a direct object. So Esperanto uses moviĝi, the intransitive form:

• moviĝas = move, are moving

So:

• veturiloj moviĝas = vehicles are moving
• veturiloj movas would suggest that the vehicles are moving something else

The element -iĝ- often makes a verb intransitive, with a sense like become or come to be, or do something oneself / be in that state.

So:

• movi = to move something
• moviĝi = to move oneself, to be moving

In this sentence, moviĝas is the present-tense form of moviĝi:

• -as = present tense

So moviĝas means move / are moving.

Because Esperanto verbs do not change for person or number.

That means the same verb form is used whether the subject is singular or plural:

• La veturilo moviĝas = The vehicle moves
• La veturiloj moviĝas = The vehicles move

This is much simpler than in English, where you get differences like moves versus move.

Pli malrapide means more slowly.

Here is the breakdown:

• rapida = fast
• malrapida = slow
• malrapide = slowly
• pli = more

So:

• pli malrapide = more slowly

It uses -e because malrapide is an adverb, and it describes how the vehicles move.

Compare:

• malrapida veturilo = a slow vehicle
• veturilo moviĝas malrapide = a vehicle moves slowly

It is not stated here, because the comparison is understood from context.

In English, we also do this sometimes:

• In fog, vehicles move more slowly.

That implies more slowly than usual or more slowly than in clear conditions.

If you want to state the comparison explicitly in Esperanto, you use ol:

• En la nebulo veturiloj moviĝas pli malrapide ol kutime.
  = In the fog, vehicles move more slowly than usual.

Yes, in many contexts that would also be possible.

• moviĝas is broad and general: move / are moving
• veturas is more specifically about traveling or driving by vehicle

So the original sentence with moviĝas is neutral and general. It focuses on motion itself.
A version with veturas would focus more on the act of traveling by vehicle.

Both can work, but moviĝas is a very straightforward choice for move.

A rough guide:

• moviĝas ≈ mo-VEE-jas
• veturiloj ≈ ve-too-REE-loy

A few useful details:

• ĝ is pronounced like the j in judge
• j at the end of a word is like an English y sound
• oj sounds like oy, as in boy

So:

• veturiloj ends with -oy
• moviĝas has ĝ, the j sound of judge