Korisnik sumnja da je reciklaža potrebna.

Serbian does not use articles.

So:

• korisnik can mean the user or a user
• reciklaža means recycling without any article

Whether English needs the, a, or no article at all depends on context, not on a separate Serbian word.

Korisnik usually means user, especially in technical, online, or service contexts.

It is:

• singular
• masculine grammatical gender
• nominative case here, because it is the subject of the sentence

If you wanted to specify a female user, you would normally say korisnica.

Here, sumnja is a verb: (he/she/it) doubts or suspects.

It comes from the verb sumnjati = to doubt / to suspect.

Why it is clearly a verb here:

• it follows the subject korisnik
• it is in the 3rd person singular present tense

Be aware that sumnja can also be a noun meaning doubt or suspicion, but not in this sentence.

Da introduces a subordinate clause. In this sentence it works like English that:

• Korisnik sumnja da...
• The user doubts/suspects that...

This is a very common Serbian pattern after verbs of thinking, saying, knowing, feeling, doubting, and similar meanings.

Because je is a clitic, and Serbian clitics usually go very early in the clause.

So the natural order is:

• da je reciklaža potrebna

not normally:

• da reciklaža je potrebna

A useful learner rule is: after words like da, short forms such as je often come immediately after them.

Both are in the nominative case.

• korisnik is nominative because it is the subject of the main clause
• reciklaža is nominative because it is the subject of the subordinate clause

So the structure is basically:

• Korisnik = subject
• sumnja = verb
• da je reciklaža potrebna = subordinate clause
• inside that clause, reciklaža = subject and potrebna = predicate adjective

Because potrebna agrees with reciklaža.

Reciklaža is:

• feminine
• singular
• nominative

So the adjective must match it:

• masculine singular: potreban
• feminine singular: potrebna
• neuter singular: potrebno

Since reciklaža is feminine, potrebna is the correct form.

No. Potrebna is an adjective meaning necessary or needed.

The verb is je = is.

So:

• reciklaža je potrebna = recycling is necessary

This is a normal Serbian pattern:

• noun + je
  • adjective

It can mean either, depending on context.

With da + clause, sumnjati often means being uncertain about whether something is true. In English, that may become:

• doubt that...
• suspect that...

Context decides which English translation sounds best.

A related pattern is sumnjati u + accusative, which often means to doubt or to have doubts about something:

• Sumnjam u to. = I doubt that / I have doubts about that.

So this is a verb where translation can shift a little depending on the situation.

Serbian word order is fairly flexible, but not completely free.

The neutral version here is:

• Korisnik sumnja da je reciklaža potrebna.

You can change parts of the subordinate clause for emphasis, for example:

• Korisnik sumnja da je potrebna reciklaža.

That can sound slightly different in focus, but the core meaning stays similar.

What usually stays important is clitic placement, especially je, which should remain near the beginning of its clause.

You would usually change korisnik to korisnica:

• Korisnica sumnja da je reciklaža potrebna.

Everything else stays the same because:

• sumnja is still 3rd person singular
• reciklaža je potrebna is unchanged

So only the noun referring to the person changes.