Kad je kvaka mokra, vrata se teže otvaraju.

Yes. Kad and kada both mean when.

In everyday Croatian, kad is very common and slightly shorter/more conversational. Kada can sound a bit more formal or simply fuller, but in a sentence like this both are natural:

• Kad je kvaka mokra, vrata se teže otvaraju.
• Kada je kvaka mokra, vrata se teže otvaraju.

Both mean the same thing here.

Because the sentence has two clauses:

• Kad je kvaka mokra = when the handle is wet
• vrata se teže otvaraju = the door opens more difficultly / is harder to open

In Croatian, a subordinate clause introduced by kad is normally separated by a comma from the main clause.

So this is standard punctuation:

• Kad je kvaka mokra, vrata se teže otvaraju.

You could also reverse the order:

• Vrata se teže otvaraju kad je kvaka mokra.

Then the comma is often omitted, just as in many similar Croatian sentences.

Because mokra agrees with kvaka.

Kvaka is a feminine singular noun, so the adjective must also be feminine singular:

• mokar = masculine singular
• mokra = feminine singular
• mokro = neuter singular

So:

• kvaka → feminine
• therefore mokra

This is basic adjective agreement in Croatian: adjectives match the noun in gender, number, and case.

This is one of the things English speakers often find surprising.

Vrata is a plural-form noun that usually refers to a door as a single object. Grammatically, though, it behaves like a plural noun.

So Croatian says:

• vrata su zatvorena = the door is closed
• vrata se otvaraju = the door opens / the doors open

Even when English uses singular door, Croatian often uses plural vrata.

That means the verb also appears in the plural:

• otvaraju = plural not
• otvara = singular

Here se helps create a meaning like:

• the door opens
• the door can be opened
• the door is opened
• the door is hard to open

In this sentence, otvarati se is not really reflexive in the English sense of open itself. Instead, it gives a kind of general, impersonal, or middle/passive meaning.

So:

• otvarati = to open something
• otvarati se = to open, to be opening, to be opened

In context, vrata se teže otvaraju is best understood as:

• the door is harder to open or
• the door doesn’t open as easily

This use of se is very common in Croatian.

Here teže means more difficultly, with more difficulty, or more naturally in English, more hard / less easily.

It is the comparative adverb related to teško:

• teško = with difficulty / hard
• teže = more difficultly / harder

So:

• Vrata se teško otvaraju. = The door opens with difficulty.
• Vrata se teže otvaraju. = The door opens with even more difficulty / is harder to open.

For learners, the most natural translation is usually:

• When the handle is wet, the door is harder to open.

Even though teže can also remind learners of težak = heavy, here the meaning is about difficulty, not weight.

Because the sentence expresses a general truth or usual result, not a one-time event.

Kad je kvaka mokra, vrata se teže otvaraju. means something like:

• Whenever the handle is wet, the door is harder to open.

Croatian often uses the present tense for this kind of general statement, just like English does in sentences such as:

• When it rains, the roads get slippery.

So otvaraju here does not mean that the door is opening right now; it describes what generally happens under those conditions.

Because otvaraju is from the imperfective verb otvarati, while otvore would come from the perfective verb otvoriti.

Croatian aspect matters a lot:

• otvarati = imperfective, process/repeated/general action
• otvoriti = perfective, completed action

In a sentence about a repeated or general situation, Croatian normally prefers the imperfective:

• Kad je kvaka mokra, vrata se teže otvaraju.

This means:

• in that condition, opening the door is generally more difficult

If you used the perfective form, the sentence would sound more like a specific completed event, which does not fit as well here.

Yes, absolutely.

Both are natural:

• Kad je kvaka mokra, vrata se teže otvaraju.
• Vrata se teže otvaraju kad je kvaka mokra.

The difference is mainly one of focus:

• starting with Kad je kvaka mokra... emphasizes the condition first
• starting with Vrata se teže otvaraju... emphasizes the result first

So the word order is flexible, as long as the sentence remains clear.

Kvaka usually means a door handle, especially the kind you press down.

Depending on context, English translations might vary a little:

• handle
• sometimes door handle
• less often something like latch

It is not usually the best word for a round doorknob in every context, though in loose translation English speakers may still say handle or knob depending on the door.

So in this sentence, door handle is the safest translation.

Sometimes kad in Croatian can feel close to whenever or even if/when, depending on context.

In this sentence:

• Kad je kvaka mokra, vrata se teže otvaraju.

the meaning is mainly when/whenever the handle is wet.

Because this is a general-condition sentence, English could also paraphrase it as:

• If the handle is wet, the door is harder to open.

So the Croatian sentence is most directly when, but the overall logic is condition-like, which is why if can sound reasonable in English translation too.

Both main nouns are in the nominative:

• kvaka = nominative singular
• vrata = nominative plural form

That is because each one is the subject of its clause:

• kvaka is the subject of je mokra
• vrata is the subject of se teže otvaraju

So the basic structure is:

• Kad je kvaka mokra = When the handle is wet
• vrata se teže otvaraju = the door opens more difficultly / is harder to open

No object case is needed here.

Yes, it is natural and understandable.

A native speaker could definitely say:

• Kad je kvaka mokra, vrata se teže otvaraju.

In everyday speech, someone might also choose a slightly different wording, for example:

• Kad je kvaka mokra, teže je otvoriti vrata.
• Kad je kvaka mokra, vrata se teže mogu otvoriti.

These alternatives may sound a bit more idiomatic to some speakers, especially if they want to stress the difficulty of opening the door. But your original sentence is still perfectly good Croatian.