Most je nad rekom.

• most = bridge
• je = is
• nad = above / over
• rekom = river in a changed form, here meaning over the river or above the river

So the sentence is literally something like Bridge is above/over river.

Because Serbian changes noun forms depending on their role in the sentence. This is called case.

The basic dictionary form is reka = river.
After the preposition nad when it describes a location or position, Serbian normally uses the instrumental case.

• reka = river
• rekom = over/above the river, after nad in this kind of sentence

So:

• nad rekom = over the river / above the river

It is the instrumental singular of reka.

Here is the pattern:

• reka → nominative singular
• rekom → instrumental singular

In this sentence, nad takes the instrumental because the sentence describes where the bridge is located.

Je is the 3rd person singular present form of the verb biti = to be.

So:

• on je = he is
• ona je = she is
• most je = the bridge is

In Serbian, the present tense of to be is usually expressed explicitly in normal statements, so je is needed here.

Normally, no in a standard sentence like this.

• Most je nad rekom. = correct
• Most nad rekom. = incomplete in standard Serbian

In some informal styles, headlines, poetry, or very compressed speech, Serbian may omit forms of to be, but for a learner, you should keep je here.

Because Serbian does not have articles like English a and the.

So most can mean:

• a bridge
• the bridge

Which one is meant depends on context.

That means Most je nad rekom could be understood as:

• The bridge is over the river or
• A bridge is over the river

In practice, context tells you which one is intended.

Yes, but they are not exactly the same.

• nad = above / over
• preko = across / over

So:

• Most je nad rekom. focuses on the bridge being above the river.
• Most je preko reke. often sounds more like the bridge goes across the river.

Both can be natural depending on what exactly you want to express, but nad rekom emphasizes position over the river.

Yes. Very literally, it means above the river.

In natural English, though, we usually say:

• The bridge is over the river

So Serbian nad often matches English over in this kind of sentence, even though above is also a close literal meaning.

No, Serbian word order is fairly flexible, though some orders sound more neutral than others.

The neutral order here is:

• Most je nad rekom.

But you could also say:

• Nad rekom je most.

That version puts more focus on where the bridge is. It can sound like Over the river is a bridge or It is over the river that the bridge is.

So the original sentence is the most straightforward, neutral version.

Most is a masculine noun.

You do not see that very clearly in this sentence, but it matters when adjectives or past-tense forms are used.

For example:

• stari most = the old bridge
• Most je star. = The bridge is old.

The adjective star is in the masculine form because most is masculine.

Yes, iznad reke is possible, but it is slightly different.

• nad rekom = over / above the river
• iznad reke = above the river

Iznad often sounds a bit more specifically spatial, as in higher than or above.
For a bridge, nad rekom is very natural.
If you said Most je iznad reke, it would also be understandable, but it may sound more like you are emphasizing the bridge’s vertical position.

Because reka belongs to a noun pattern where many feminine nouns ending in -a form the instrumental singular with -om or -em.

So:

• žena → ženom
• škola → školom
• reka → rekom

The stem can sometimes change slightly depending on the word, and reka → rekom is the normal correct form you should learn as part of the noun’s case pattern.