Opasna materija je u opasnom otpadu.

Because Serbian adjectives must agree with the noun they describe in gender, number, and case.

• materija is feminine singular nominative
  • so the adjective is opasna
• otpad is masculine singular
  • and after u here, it is in the locative case
  • so the adjective becomes opasnom

So:

• opasna materija = feminine singular nominative
• u opasnom otpadu = masculine singular locative

This kind of adjective agreement is very important in Serbian.

Because after the preposition u meaning in, Serbian usually uses the locative case when talking about location.

The dictionary form is:

• otpad = waste

But in the locative singular, it becomes:

• u otpadu = in the waste

Since the noun changes case, the adjective changes too:

• u opasnom otpadu = in dangerous waste

So otpadu is simply the locative form of otpad.

It is in the nominative singular.

Why? Because materija is the subject of the sentence:

• Opasna materija = the dangerous substance/material
• je = is
• u opasnom otpadu = in dangerous waste

In other words, the sentence is built like:

• [subject] + [is] + [location]

So the subject materija stays in the nominative.

Because here u means in in the sense of location: something is already somewhere.

• je u opasnom otpadu = is in dangerous waste

With u, Serbian usually works like this:

• u + locative = location, being inside something
• u + accusative = motion into something

Compare:

• Materija je u otpadu. = The substance is in the waste.
  
  • location → locative
• Stavljaju materiju u otpad. = They are putting the substance into the waste.
  
  • motion/direction → accusative

This is a very common pattern in Serbian.

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:

• Opasna materija je u opasnom otpadu.
• literally: Dangerous material is in dangerous waste.

So je is just the Serbian word for is.

Usually, in a normal full sentence like this, no—you keep it.

• Opasna materija je u opasnom otpadu. = correct standard sentence

If you remove je, the sentence sounds incomplete in standard Serbian:

• Opasna materija u opasnom otpadu. = not a normal full sentence; it sounds more like a label, note, or headline

Serbian sometimes drops forms of to be in informal styles, headlines, or very compressed writing, but in standard grammar here, je should stay.

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

So:

• materija can mean a substance/material or the substance/material
• otpad can mean waste or the waste, depending on context

Serbian speakers understand definiteness from:

• context
• word order
• emphasis
• the situation

That means a sentence like this can be translated in more than one natural English way depending on context.

No, Serbian word order is fairly flexible.

The sentence as given:

• Opasna materija je u opasnom otpadu.

is a neutral, natural order.

But Serbian can move parts around for emphasis or style, for example:

• U opasnom otpadu je opasna materija.
  This puts more focus on where it is.

Because Serbian uses case endings, the grammatical roles are still clear even when word order changes.

That said, not every order sounds equally neutral. The original sentence is a straightforward default-style sentence.

Not exactly.

Although materija is related in form to English matter/material, in Serbian it often means:

• substance
• material
• matter in a scientific or technical sense

In this kind of sentence, depending on context, English might translate it as:

• dangerous substance
• hazardous material

So it is a cognate-like word, but you should not assume it always matches English matter perfectly in every context.

In this sentence:

• materija is feminine
• otpad is masculine

A useful beginner clue is the ending:

• nouns ending in -a are often feminine
  
  • materija
• many nouns ending in a consonant are often masculine
  
  • otpad

This is not a perfect rule, but it works often enough to be helpful.

You can also see the gender from adjective agreement:

• opasna materija → feminine adjective form
• opasnom otpadu → masculine adjective form in locative

A rough pronunciation guide is:

• Opasna materija je u opasnom otpadu
• approximately: oh-PAHS-nah mah-teh-REE-yah yeh oo oh-PAHS-nom OHT-pah-doo

A few useful pronunciation notes:

• j is pronounced like English y
  • je sounds like ye
  • materija has -rija, roughly -ree-ya
• č, ć, š, ž are not in this sentence, so pronunciation is fairly straightforward
• Serbian spelling is quite phonetic, so words are usually pronounced close to how they are written

Yes. In technical, legal, environmental, or safety contexts, opasan is often best translated as:

• dangerous
• hazardous

So:

• opasna materija can be dangerous substance or hazardous material
• opasan otpad can be dangerous waste or hazardous waste

The best English choice depends on the context, especially whether the text is everyday language or technical/regulatory language.