Opasan otpad ide na odvojeno sakupljanje.

Because otpad is a masculine singular noun, and here the adjective appears in the short/basic masculine nominative singular form: opasan.

In Serbian, some masculine adjectives can have both a short form and a long form in the nominative singular:

• opasan
• opasni

In a sentence like this, opasan otpad is the normal generic expression meaning hazardous waste.
Opasni otpad can also exist, but it often sounds more definite, more specific, or just stylistically different depending on context.

So for a general label or rule, opasan otpad is the expected form.

It is in the nominative case, because it is the subject of the sentence.

Structure:

• Opasan otpad = subject
• ide = verb
• na odvojeno sakupljanje = destination/purpose phrase

So literally, the sentence is built like:

Hazardous waste + goes + to separate collection

In Serbian, otpad is usually a singular mass/collective noun, much like waste in English.

So even though it may refer to many pieces or types of waste, Serbian normally says:

• otpad = waste

not a plural form in this kind of statement.

That is why opasan is also singular, agreeing with otpad.

Literally, yes: ide means goes.

But in this kind of practical, instructional, or label-style sentence, ide often means something like:

• goes into
• belongs in
• should be put in
• is sent for

So Opasan otpad ide na odvojeno sakupljanje does not mean the waste is physically walking somewhere. It means:

• hazardous waste belongs in separate collection
• hazardous waste should be collected separately

This is a very natural Serbian way to express sorting or disposal instructions.

The preposition na here expresses a kind of destination, placement, or procedure.

With verbs like ići (to go), Serbian often uses:

• na + accusative

to show where something goes.

So:

• ide na odvojeno sakupljanje = goes for separate collection

It is similar in logic to expressions like:

• ide na popravku = it goes for repair
• ide na pregled = it goes for examination

Here, na is not really just physical location; it often means for / into the process of.

It is in the accusative, because it follows na with the idea of movement/destination.

So:

• ići na + accusative = to go to / for / into

The noun sakupljanje is neuter singular, and in neuter singular, the nominative and accusative forms are the same:

• nominative: odvojeno sakupljanje
• accusative: odvojeno sakupljanje

So the form does not change visibly, but grammatically it is accusative here.

Here it is an adjective, not an adverb.

It agrees with sakupljanje, which is a neuter singular noun:

• odvojeno sakupljanje = separate collection

Why it may look confusing:

• as an adverb, odvojeno can mean separately
• as a neuter singular adjective, odvojeno can mean separate

In this sentence, it modifies the noun sakupljanje, so it is functioning as an adjective.

Compare:

• Otpad se sakuplja odvojeno. = The waste is collected separately.
  
  • Here odvojeno is an adverb.
• Otpad ide na odvojeno sakupljanje. = The waste goes for separate collection.
  
  • Here odvojeno is an adjective.

Sakupljanje is a verbal noun formed from the verb sakupljati / sakupiti (to collect, gather).

So:

• sakupljati = to collect
• sakupljanje = collecting / collection

In this sentence, it refers to the process or system of collection.

So odvojeno sakupljanje means:

• separate collection
• separate collecting
• in natural English, often separate waste collection

Yes. That is a very natural interpretation.

Serbian often uses the present tense in signs, notices, rules, and instructions where English might use:

• should
• must
• is to be
• passive constructions

So although the Serbian sentence is literally Hazardous waste goes for separate collection, the practical meaning is often:

• Hazardous waste should be collected separately
• Hazardous waste belongs in separate collection
• Dispose of hazardous waste separately

Because Serbian does not have articles.

So opasan otpad can mean, depending on context:

• hazardous waste
• the hazardous waste
• a hazardous waste type (less likely here)

You understand the intended meaning from context, not from an article.

That is one reason Serbian sentences can look more compact than English ones.

Yes, Serbian word order is fairly flexible.

The neutral order here is:

• Opasan otpad ide na odvojeno sakupljanje.

But other orders are possible for emphasis, for example:

• Na odvojeno sakupljanje ide opasan otpad.

That version puts more focus on where it goes.
Still, the original sentence is the most natural neutral wording for a rule or notice.

Yes. A very common alternative would be:

• Opasan otpad se sakuplja odvojeno.

That means:

• Hazardous waste is collected separately.

Compare the two:

• Opasan otpad ide na odvojeno sakupljanje.
  • focuses on the category/disposal route
  • literally goes to separate collection
• Opasan otpad se sakuplja odvojeno.
  • focuses on the action
  • is collected separately

Both are natural, but the original sounds especially suitable for sorting instructions, recycling rules, or waste-disposal labels.