Spust je klizav zbog blata.

Spust is in the nominative singular because it is the subject of the sentence—the thing being described.

• spust = the slope / the descent
• je klizav = is slippery

So the structure is basically:

• [subject] + [to be] + [adjective] + [reason phrase]

Because klizav agrees with spust.

Since spust is:

• masculine
• singular
• nominative

the adjective also has to be:

• masculine
• singular
• nominative

That is why you get klizav.

Compare:

• spust je klizav — the slope is slippery
• staza je klizava — the path is slippery
• tlo je klizavo — the ground is slippery

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

So here it simply means is.

• Spust je klizav = The slope is slippery

Unlike in some other Slavic languages, Serbian normally does use the present-tense form of to be in ordinary sentences like this.

Because je is a clitic in Serbian. Clitics usually go in the second position in the sentence or clause.

So in:

• Spust je klizav zbog blata.

the first element is Spust, and je comes right after it.

This is very normal Serbian word order. You will see this a lot with forms like:

• je
• sam
• si
• su
• short pronouns such as ga, mu, se

Zbog means because of, due to, or on account of.

It introduces the reason for something:

• zbog blata = because of the mud
• zbog kiše = because of the rain
• zbog snega = because of the snow

It is a very common preposition when you want to give a cause or reason.

Because zbog requires the genitive case.

The noun is:

• blato = mud

But after zbog, it changes to genitive:

• zbog blata = because of the mud

So:

• blato = nominative
• blata = genitive

This is one of the most important things to notice in the sentence: prepositions in Serbian often determine the case of the noun that follows.

Yes, zbog is followed by the genitive.

Examples:

• zbog blata — because of the mud
• zbog kiše — because of the rain
• zbog problema — because of the problem
• zbog dece — because of the children

So if you learn zbog, it is a good idea to learn it together with the rule:

• zbog + genitive

Yes, Serbian word order is fairly flexible, although some versions sound more neutral than others.

The original:

• Spust je klizav zbog blata.

Other possible versions:

• Zbog blata je spust klizav.
• Spust je zbog blata klizav.

These all mean roughly the same thing, but the emphasis changes a little.

A very important point: even when the word order changes, je still tends to stay in second position.

Not directly in the same structure.

• zbog is a preposition, so it is followed by a noun phrase
• jer means because, but it introduces a whole clause

So you can say:

• Spust je klizav zbog blata.
  = The slope is slippery because of the mud.

But with jer, you need something more like:

• Spust je klizav jer na njemu ima blata.
• Spust je klizav jer je pun blata.

So zbog and jer are related in meaning, but they are not used in exactly the same grammatical way.

Because Serbian has no articles like English a and the.

So spust can mean:

• a slope
• the slope

The context tells you which one is meant. In this sentence, English would usually translate it as the slope because it sounds like a specific slope already known in the situation.

In Serbian Cyrillic, it is:

Спуст је клизав због блата.

Serbian uses both Latin and Cyrillic, and both are standard.