On vozi pažljivije kad pada kiša.

Because pažljivije is the comparative form of the adverb pažljivo.

• pažljivo = carefully
• pažljivije = more carefully

So the sentence means that he drives more carefully when it rains, not just carefully in general.

This is similar to English:

• carefully → more carefully

In Croatian, many adverbs form the comparative with -ije or -e.

Vozi is the 3rd person singular present tense of voziti.

So:

• ja vozim = I drive / I am driving
• ti voziš = you drive
• on vozi = he drives / he is driving

In this sentence, vozi is best understood as a habitual present:

• He drives more carefully when it rains.

Croatian present tense often covers both ideas that English separates into:

• he drives
• he is driving

The context tells you which meaning is intended.

Croatian often uses the present tense for general truths, habits, and repeated situations.

So:

• On vozi pažljivije kad pada kiša. literally uses present tense in both clauses, but it means:
• He drives more carefully when it rains.

This does not mean only one specific moment right now. It usually describes a regular pattern.

The same happens in English:

• I stay home when it rains. Not necessarily right now, but generally.

Yes. Here kad means when.

In this sentence, it introduces a time clause:

• kad pada kiša = when it rains

A useful detail:

• kad is very common in everyday speech
• kada is the fuller form, often a bit more formal or careful

So both are possible:

• On vozi pažljivije kad pada kiša.
• On vozi pažljivije kada pada kiša.

They mean the same thing here.

Yes, literally it is something like rain falls, but this is the normal Croatian way to say it is raining or rain falls.

So:

• Pada kiša. = It is raining.
• Kad pada kiša = When it rains.

English uses a dummy subject:

• it rains

Croatian does not need that kind of it here. Instead, kiša is the real subject.

Both word orders are possible.

• kad pada kiša
• kad kiša pada

Croatian word order is more flexible than English word order. The version with pada kiša sounds very natural and common.

The difference is usually not in basic meaning, but in focus, rhythm, or style.

In neutral speech, kad pada kiša is a very normal choice.

Yes, very often you can.

Croatian usually does not need subject pronouns when the verb already makes the person clear.

So:

• Vozi pažljivije kad pada kiša.

can also mean:

• He drives more carefully when it rains.

Why include on then?

Usually for:

• emphasis
• contrast
• clarity

For example:

• On vozi pažljivije, a ona ne. = He drives more carefully, but she doesn’t.

So in many everyday contexts, on would be optional.

Kiša is in the nominative singular because it is the subject of pada.

Here the structure is:

• kiša pada = rain falls / it rains

So:

• kiša = the thing doing the action grammatically
• therefore it stays in the nominative

There is no special case after kad here, because kad introduces a whole clause, not a noun phrase.

Here it is an adverb.

It describes how he drives:

• vozi pažljivije = he drives more carefully

Compare:

• pažljiv = careful
  adjective, used for a noun

  • On je pažljiv vozač. = He is a careful driver.

• pažljivo = carefully
  adverb, used for a verb

  • On vozi pažljivo. = He drives carefully.

• pažljivije = more carefully
  comparative adverb

  • On vozi pažljivije. = He drives more carefully.

So in your sentence, it modifies the verb vozi.

Vozi comes from voziti, which is imperfective.

That matters because the sentence describes:

• an ongoing ability or activity
• a repeated habit
• a general pattern

Imperfective verbs are the normal choice for that.

Likewise, pada comes from padati, also imperfective, because rain here is viewed as an ongoing or repeated event:

• kad pada kiša = when it is raining / when it rains

If you are still early in learning Croatian, the main thing to remember is:

• imperfective is very common for habits and ongoing situations
• this sentence is a good example of that

Sometimes learners wonder about this because both can relate to time, but here kad is the most natural choice.

• kad = when
• dok = while, during the time that

Your sentence expresses a general condition/time relationship:

• He drives more carefully when it rains.

So kad fits very well.

If you used dok, it would sound more like:

• while it is raining

That can work in some contexts, but it is not the most natural equivalent for this general statement.

So for this sentence, stick with:

• kad pada kiša