Oblaci su nad marinom.

Because oblaci is the plural form of oblak (cloud).

• oblak = cloud
• oblaci = clouds

So the sentence starts with a plural subject, which is why Croatian also uses the plural verb su (are).

A small spelling change happens too:

• oblak → oblaci

The k changes to c in this plural form. This is a normal pattern in Croatian for some nouns.

Su means are.

It is the 3rd person plural present form of the verb biti (to be).

A few useful forms of biti in the present tense are:

• sam = I am
• si = you are
• je = he/she/it is
• smo = we are
• ste = you are
• su = they are

Since oblaci is plural (clouds), Croatian uses su.

Because after nad in this sentence, Croatian uses the instrumental case.

So:

• marina = base form / dictionary form
• marinom = instrumental singular

The sentence describes location: the clouds are over / above the marina, not moving toward it. With nad used for location, the noun normally goes into the instrumental.

So:

• nad marinom = over the marina / above the marina

Nad means over or above.

It is used when something is positioned higher than something else.

So:

• nad marinom = above the marina / over the marina

This is a very common spatial preposition in Croatian.

This is a very common learner question.

Both can often translate as above or over, but they are not always used in exactly the same way.

• nad often expresses a spatial relationship more directly, and it works with cases:
  • instrumental for location
  • accusative for movement
• iznad usually means above, often in a more neutral sense of higher than, and it takes the genitive

Examples:

• Oblaci su nad marinom. = The clouds are over the marina.
• Oblaci su iznad marine. = The clouds are above the marina.

Both can be possible, but nad marinom feels very natural for physical position overhead.

This is the normal, neutral word order:

• Oblaci = subject
• su = verb
• nad marinom = prepositional phrase

So it is basically:

Clouds are over the marina.

Croatian word order is more flexible than English, so other orders are possible, for example:

• Nad marinom su oblaci.

This can sound more focused on where the clouds are.

One important thing to notice: su is a clitic, and clitics usually appear in the second position of the sentence or clause. That affects word order a lot in Croatian.

Normally, in a sentence like this, no.

Croatian often drops subject pronouns, but it usually does not drop the verb to be in this kind of ordinary present-tense statement.

So:

• Oblaci su nad marinom. = correct
• Oblaci nad marinom. = not a normal full sentence in standard usage; it sounds more like a caption or headline

With nad, Croatian often uses:

• instrumental for location
• accusative for movement toward a position above something

In your sentence, it is location:

• Oblaci su nad marinom. = The clouds are over the marina.

If you wanted movement to a position above the marina, you could use accusative:

• Oblaci dolaze nad marinu. = The clouds are moving over/toward the marina.

So the case changes depending on whether the sentence describes being somewhere or moving somewhere.

Yes, marina is used in Croatian and means marina in the nautical sense.

It is a feminine noun, which is why its singular forms follow a common feminine pattern:

• marina = nominative
• marine = genitive
• marini = dative/locative
• marinu = accusative
• marinom = instrumental

That is why after nad you get marinom.

A simple approximate pronunciation for an English speaker is:

OB-lah-tsee soo nahd mah-REE-nom

A few useful pronunciation notes:

• c in Croatian is pronounced like ts
• č and ć are different sounds, but there is no č/ć here
• r is trilled or tapped
• stress is not usually marked in normal writing

So oblaci sounds roughly like oblah-tsee, not like an English c sound.

Yes, it is grammatical and natural.

It sounds like a normal descriptive sentence, especially in contexts like:

• weather description
• talking about a view
• describing a photograph or scene

A native speaker might also say:

• Nad marinom su oblaci.
• Iznad marine su oblaci.

But your original sentence is perfectly normal Croatian.