Teorijska nastava je važna.

A word-by-word breakdown is:

• teorijska = theoretical
• nastava = teaching, instruction, or classes depending on context
• je = is
• važna = important

So the sentence literally reads: Theoretical instruction is important or Theoretical classes are important.

Because it agrees with nastava, which is a feminine singular noun.

In Serbian, adjectives change their form to match the noun in:

• gender
• number
• case

Since nastava is feminine singular, the adjective must also be feminine singular:

• masculine: teorijski
• feminine: teorijska
• neuter: teorijsko

So teorijska nastava is the correct matching form.

For the same reason: važna describes nastava, so it must agree with it.

Even though važna comes after je and works like important in English, Serbian still makes it match the subject:

• nastava je važna = instruction is important
• kurs je važan = the course is important
• predavanje je važno = the lecture is important

So:

• važan = masculine
• važna = feminine
• važno = neuter

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

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In this sentence, je means is:

• Teorijska nastava je važna = Theoretical instruction is important

Normally, no. In standard Serbian, you usually keep je in this kind of sentence.

So:

• Teorijska nastava je važna = correct
• Teorijska nastava važna = not standard as a normal full sentence

This is different from some other Slavic languages where the present-tense to be may be omitted.

It is in the nominative singular.

Why? Because nastava is the subject of the sentence:

• Teorijska nastava = the thing being talked about
• je važna = is important

The adjective teorijska is also nominative singular feminine because it modifies nastava.

Because Serbian does not have articles like English a/an/the.

So nastava can mean:

• teaching
• the teaching
• instruction
• the instruction

The exact meaning depends on context.

That means Teorijska nastava je važna could be understood in English as:

• Theoretical instruction is important
• The theoretical instruction is important
• Theoretical classes are important

Not exactly. Nastava is a broader word.

It often means:

• teaching
• instruction
• school classes
• coursework/classroom instruction

It does not usually mean just one single lesson in the simplest sense. Depending on context, it can refer to the instructional part of education as a whole.

For example, teorijska nastava often refers to the theoretical part of instruction/classes, especially as opposed to practical training.

Yes. Serbian word order is more flexible than English.

The neutral, standard order is:

• Teorijska nastava je važna.

But you can also change the order for emphasis, for example:

• Važna je teorijska nastava. = puts more emphasis on important
• Teorijska nastava važna je = possible in special styles, but not the most natural neutral order

For a learner, the best default is:

• subject + je + adjective
• Teorijska nastava je važna.

A rough pronunciation guide is:

teh-oh-REE-y-ska NAH-sta-va yeh VAZH-na

A few useful sound notes:

• j sounds like English y in yes
• š sounds like sh in ship
• ž sounds like the s in measure
• č / ć are different sounds in Serbian, but they do not appear in this sentence

So:

• teorijska has j = y
• je sounds like yeh
• važna begins with vazh-

In Cyrillic, it is:

Теоријска настава је важна.

Serbian uses both:

• Latin script
• Cyrillic script

They are both standard, and this sentence means exactly the same in either script.