Nadmorska visina grebena je velika.

Because Serbian uses the genitive case where English often uses of.

• greben = ridge
• grebena = of the ridge

So nadmorska visina grebena literally works like the elevation of the ridge.

The subject is nadmorska visina.

A good way to tell is by looking at the predicate adjective:

• visina is feminine singular
• so the adjective is velika = feminine singular

That means the sentence is saying something about the elevation, not directly about the ridge.

Because they both agree with visina, which is a feminine singular noun.

In Serbian, adjectives must agree with the noun they describe in:

• gender
• number
• case

So here:

• nadmorska visina = feminine singular
• visina je velika = feminine singular again

That is why both adjectives have the -a ending.

Yes. Nadmorska visina is a standard expression meaning altitude / elevation above sea level.

It is best learned as a single unit.

Literally, the parts are:

• nad- = above
• morska = sea-related
• visina = height

But in normal use, the whole phrase simply means elevation above sea level.

Je is the 3rd person singular present tense of biti (to be).

So:

• je velika = is high / is great / is considerable

In standard Serbian, you normally keep je in sentences like this. Unlike in some other Slavic languages, Serbian usually does not drop the present-tense form of to be in ordinary statements.

Because je is a clitic, and Serbian clitics usually stand in the second position of the clause.

Here, the first whole unit is:

• Nadmorska visina grebena

So the clitic je comes after that unit:

• Nadmorska visina grebena je velika.

You do not normally split that noun phrase just to place je earlier.

Yes, visoka can also be used with nadmorska visina.

• velika = great, considerable, large
• visoka = high

In this sentence, velika means something like considerably high or great. It is natural Serbian.
Visoka would also be understandable and natural in many contexts, but velika is perfectly fine here.

So the sentence is not strange; it is just using velik in the sense of considerable / high in degree.

Because Serbian has no articles.

So Serbian does not have direct equivalents of English a and the in normal grammar.

Whether something is definite or indefinite is usually understood from:

• the context
• word order
• the situation

So grebena can be understood as of the ridge from context, even though there is no separate word for the.

Not really. This sentence specifically talks about the ridge's altitude above sea level, not its own vertical size.

• Nadmorska visina grebena je velika = the ridge is at a high elevation
• Greben je visok = the ridge itself is tall/high

So the phrase nadmorska visina makes the meaning much more specific.

Yes. A very natural alternative is:

• Greben je na velikoj nadmorskoj visini.
• Greben se nalazi na velikoj nadmorskoj visini.

These mean essentially the same thing, but the focus shifts a little:

• Nadmorska visina grebena je velika. → focus on the elevation
• Greben je na velikoj nadmorskoj visini. → focus on the ridge

Both are good Serbian.