Kad ruža procvjeta, vrt izgleda ljepše.

Kad means when.

Yes, kad and kada both mean when. The difference is mostly style:

• kad = very common, especially in everyday speech
• kada = a bit fuller, sometimes slightly more formal or emphatic

So this sentence could also be:

Kada ruža procvjeta, vrt izgleda ljepše.

Both are correct.

This is a very common Croatian pattern.

Procvjeta is the present form of a perfective verb: procvjetati. Perfective verbs often use present-tense forms to refer to a future completed event, especially after words like:

• kad = when
• ako = if
• čim = as soon as

So:

• Kad ruža procvjeta = When the rose blooms / once the rose comes into bloom

In English, this is actually similar to When the rose blooms, not When the rose will bloom.

The main difference is aspect and focus.

• procvjetati / procvjeta = to come into bloom, to start blooming, to bloom fully
  This focuses on the change or the moment the flower reaches bloom.
• cvjetati / cvjeta or cvasti / cvate = to be blooming / to bloom
  This focuses more on the ongoing state or repeated action.

So:

• Kad ruža procvjeta = when the rose comes into bloom
• Kad ruža cvjeta / cvate = when the rose is blooming

In your sentence, procvjeta fits well because it marks the point after which the garden looks nicer.

Because ruža is the subject of the first clause.

In Croatian, the subject is normally in the nominative case. The nominative singular of ruža is:

• ruža = rose

If it were a direct object, you would expect a different form, such as ružu in the accusative.

Here:

• ruža procvjeta = the rose blooms

So ruža stays in the nominative.

For the same reason: vrt is the subject of the second clause.

• vrt izgleda ljepše = the garden looks nicer

The nominative singular of vrt is simply vrt, so there is no ending change here.

Because Croatian does not have articles like English the and a/an.

So:

• ruža can mean a rose or the rose
• vrt can mean a garden or the garden

Context tells you which meaning is intended. In a sentence like this, English often uses the:

• When the rose blooms, the garden looks nicer

But Croatian does not need separate article words.

Because ljepše is the comparative form: nicer / more beautifully.

Compare:

• lijepo = nicely, beautifully
• ljepše = more nicely, more beautifully, nicer

So:

• Vrt izgleda lijepo = The garden looks beautiful / nice
• Vrt izgleda ljepše = The garden looks nicer

The sentence is comparing the garden’s appearance to how it looked before, or to some less beautiful state.

Here it is best understood as an adverbial comparative used with izgleda.

Croatian often uses forms like this after izgledati:

• Izgleda lijepo.
• Izgleda ljepše.

A useful thing to notice is this: if it were an adjective agreeing with vrt (masculine singular), you would expect ljepši, not ljepše.

So in this sentence:

• vrt izgleda ljepše = the garden looks nicer

That -e form is what makes it especially natural here.

Because the sentence begins with a subordinate time clause:

• Kad ruža procvjeta = when the rose blooms

Then comes the main clause:

• vrt izgleda ljepše = the garden looks nicer

When the kad clause comes first, Croatian normally separates it with a comma.

So:

• Kad ruža procvjeta, vrt izgleda ljepše.

If you reverse the order, many people would write:

• Vrt izgleda ljepše kad ruža procvjeta.

Yes.

You can also say:

Vrt izgleda ljepše kad ruža procvjeta.

The meaning stays basically the same. The difference is emphasis:

• Kad ruža procvjeta, vrt izgleda ljepše.
  This puts the time condition first.
• Vrt izgleda ljepše kad ruža procvjeta.
  This starts with the main idea: the garden looks nicer.

Both are natural.

Because it describes a general or regular result.

• izgledati = to look, to appear

Here, izgleda means something like looks in a general truth or repeated situation:

• whenever the rose blooms, the garden looks nicer

So the sentence is not talking about only one exact moment. It sounds like a normal observation or general statement.

It can most naturally be understood as a general truth or a habitual observation:

• whenever the rose blooms, the garden looks nicer

But depending on context, it could also refer to a specific future occasion:

• when this rose blooms, the garden will look nicer

Croatian often allows both readings, and context decides which one is meant.