Learn Hardcore Croatian: Pod je mokar kad pada kiša.

Here, Pod je mokar = The floor is wet.
You don’t need a preposition like na (“on”) because you’re talking about the floor itself, not about something being on the floor.

Why is there no word for “the” in Pod je mokar?

Croatian has no articles (no words for “a / an / the”).
The idea of “the” is understood from context.

Pod je mokar. → The floor is wet.
Vidim pod. → I see the floor.

Whether English uses a or the is decided when translating; Croatian just uses the bare noun pod.

Why is it je mokar and not just mokar? Is je necessary?

Yes, je is necessary here.

je is the 3rd person singular form of biti (“to be”) in the present tense and works like English “is”:

Pod je mokar. = The floor is wet.
Kiša je jaka. = The rain is strong.

Leaving je out (Pod mokar) is ungrammatical in standard Croatian.

Can the word order change? For example, can I say Kad pada kiša, pod je mokar or Pod je mokar kad kiša pada?

Yes, Croatian word order is more flexible than English, and all of these are possible:

Pod je mokar kad pada kiša.
Kad pada kiša, pod je mokar.
Pod je mokar kad kiša pada.
Kad kiša pada, pod je mokar.

The meaning is the same: The floor is wet when it rains.

Differences are mostly in focus and style:

Starting with Kad… puts more focus on the condition (when it rains).
Leaving pada kiša as in the original is the most neutral, everyday word order.

Why is it kad and not kada? Are they different?

kad and kada mean the same thing: “when”.

kad is the short, more colloquial form,
kada is the full form, slightly more formal or careful.

You can use either here:

Pod je mokar kad pada kiša.
Pod je mokar kada pada kiša.

Both are correct. In speech, kad is very common.

What tense is pada and what does it tell us about time?

pada is present tense of padati (to fall) and it’s imperfective.

Here, present tense expresses a general truth / habitual situation, not necessarily something happening right now:

Pod je mokar kad pada kiša.
= Whenever it rains, the floor is (usually / always) wet.

This is similar to English “The floor gets wet when it rains”, where the present tense can also describe a general rule.

Why is the adjective mokar and not some other form like mokra or mokro?

In Croatian, adjectives agree with the gender, number, and case of the noun they describe.

pod is a masculine singular noun in nominative.
So the adjective must be masculine singular nominative: mokar.

Compare:

pod (m.) → Pod je mokar. (The floor is wet.)
stolica (f.) → Stolica je mokra. (The chair is wet.)
more (n.) → More je mokro. (The sea is wet.)

Could the sentence be Pod je mokar kad kiša pada instead of kad pada kiša?

Yes. Both are correct:

kad pada kiša
kad kiša pada

The difference is only in word order.
Croatian allows both verb–subject and subject–verb order in such clauses, especially in spoken language.

kad pada kiša sounds a bit more neutral and is slightly more common in this exact phrase, but kad kiša pada is also natural.

Is there a difference between mokar je pod kad pada kiša and Pod je mokar kad pada kiša?

Yes, there is a difference in word order and emphasis:

Pod je mokar kad pada kiša. → neutral, normal statement.
Mokar je pod kad pada kiša. → unusual; sounds poetic or strongly emphasizing mokar (“it’s WET, the floor, when it rains”).

In everyday speech, stick to Pod je mokar…. Moving je and mokar in front of pod is mostly for style (poetry, emphasis).

Could I use the future tense to say something like The floor will be wet when it rains?

Yes, you can use the future tense in Croatian if you want to talk about a specific future situation:

Pod će biti mokar kad bude padala kiša.
= The floor will be wet when it rains.

But for a general, always-true statement, Croatian strongly prefers the present tense:

Pod je mokar kad pada kiša.

So the original sentence is talking about a general rule, not one particular future event.