U ormaru su pantalone, a na polici su knjige koje često čitam.

Because they are in the locative case, which Serbian uses after u and na when talking about a location.

• u ormaru = in the closet / wardrobe
• na polici = on the shelf

Compare:

• u ormaru = in the closet

• u ormar = into the closet

• na polici = on the shelf
• na policu = onto the shelf

So the sentence is describing where things are, not where they are going.

Serbian word order is much more flexible than English word order. Starting with U ormaru puts the location first, as if setting the scene:

• U ormaru su pantalone = In the closet are the pants
• Pantalone su u ormaru = The pants are in the closet

Both are grammatical. The version in your sentence sounds natural when the speaker wants to focus on where things are.

Because su is a clitic. In Serbian, clitics usually go in the second position of the clause.

So in:

• U ormaru su pantalone
• na polici su knjige

the first element is the location phrase (U ormaru, na polici), and su comes right after it.

This is a very common Serbian pattern.

Because there are two coordinated clauses joined by a:

1. U ormaru su pantalone
2. na polici su knjige koje često čitam

Each clause has its own subject and its own verb, so each one needs su.

Pantalone is grammatically plural. Serbian treats pants/trousers as a plural noun, much like English often does with pants or trousers.

So you say:

• Pantalone su... = not je
• ove pantalone
• nove pantalone

It is a plural-only noun in normal usage.

Serbian usually does not need a special expression like English there is / there are.

Instead, it simply says:

• U ormaru su pantalone
  literally, something like In the closet are pants

That is a normal Serbian way to express existence/location.

A is a conjunction that often means something like:

• and
• while
• whereas
• and as for

In this sentence, it links two facts with a slight contrast of location:

• In the closet are pants, and on the shelf are books...

So a is not exactly the same as i.

• i is a simple and
• a often connects two parallel or contrasting pieces of information

Koje is the relative pronoun meaning which or that.

It refers back to knjige:

• knjige koje često čitam = books that I often read

Its form is determined by two things:

1. It agrees with knjige in gender and number

   • knjige is feminine plural

2. Its case depends on its role in the relative clause

   • here it is the object of čitam, so it is in the accusative

For feminine plural, nominative and accusative look the same here, so you get koje.

Because Serbian verbs usually already show the subject.

Čitam means I read / I am reading, so ja is not necessary.

You can add ja only for emphasis, contrast, or clarity:

• koje često čitam = neutral, normal
• koje ja često čitam = emphasizes I

In ordinary Serbian, the subject pronoun is often omitted.

Because često means often, so the action is habitual / repeated.

The imperfective verb čitati is the natural choice for repeated or ongoing actions:

• često čitam = I often read

A perfective verb such as pročitati would suggest finishing something, not simply reading it in a general habitual sense.

Because koje already does that job.

In:

• knjige koje često čitam

the word koje already stands for the books inside the relative clause. So adding another object pronoun would be unnecessary and ungrammatical in standard Serbian.

It works like English:

• the books that I often read

You would not normally say:

• the books that I often read them

The same idea applies here.

Because Serbian has no articles like English the and a/an.

So nouns like ormaru, polici, pantalone, and knjige appear without articles. Whether the meaning is definite or indefinite is understood from context.

For example:

• u ormaru su pantalone could mean there are pants in the closet or the pants are in the closet, depending on context
• na polici su knjige could mean there are books on the shelf or the books are on the shelf

Serbian relies much more on context than English does for this.