Droga je opasna.

Usually no. In Croatian, droga most often means an illegal drug / narcotic or a recreational drug, not just any drug or medicine.

If you mean medicine, Croatian more commonly uses:

• lijek = medicine, drug, remedy

So Droga je opasna. is understood more like Drugs are dangerous or A narcotic is dangerous, not Medicine is dangerous.

It is singular.

• droga = singular
• droge = plural

Even though it is singular, Croatian often uses the singular for a general statement about a whole category. So Droga je opasna. can express a general truth, much like English sometimes says A tiger is dangerous or Smoking is harmful.

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

So the structure is:

• Droga = subject
• je = is
• opasna = dangerous

Literally: Drug is dangerous / The drug is dangerous / Drugs are dangerous, depending on context.

Because the adjective must agree with the noun.

Droga is feminine singular, so the adjective also has to be feminine singular:

• masculine: opasan
• feminine: opasna
• neuter: opasno

Since droga is feminine, you say:

• Droga je opasna.

not:

• Droga je opasan.

A very common clue in Croatian is the ending -a.

Many nouns ending in -a are feminine, and droga is one of them.

That is why it takes:

• feminine adjective: opasna
• feminine forms in other parts of grammar too

There are some exceptions in Croatian, but for droga, feminine is correct.

It is in the nominative singular because it is the subject of the sentence.

The basic dictionary form is also nominative singular:

• droga

In Droga je opasna., both droga and opasna are in the nominative because opasna is a predicate adjective agreeing with the subject.

In normal standard Croatian, no.

You should say:

• Droga je opasna.

Leaving out je would sound incomplete in ordinary speech. You might see omitted forms in:

• headlines
• notes
• poetry
• very informal or stylized language

But for standard learning purposes, keep je.

Yes, Croatian word order is fairly flexible, but the neutral order here is:

• Droga je opasna.

You could also hear:

• Opasna je droga.

That version changes the focus/emphasis. It can sound more marked, as if emphasizing dangerous or contrasting droga with something else.

For beginners, the safest default is:

• Droga je opasna.

It is pronounced roughly like yeh.

So the sentence sounds approximately like:

• DRO-ga yeh o-PAS-na

A few pronunciation notes:

• j in Croatian is like English y in yes
• g is always a hard g
• r is rolled or tapped more than in English

Croatian has no articles like a/an/the.

So Droga je opasna. does not explicitly mark that difference the way English does. The exact English translation depends on context:

• The drug is dangerous.
• A drug is dangerous.
• Drugs are dangerous.

In many contexts, this sentence is understood as a general statement, so English often translates it naturally as:

• Drugs are dangerous.

Yes. That is the plural version:

• Droge = drugs
• su = are
• opasne = dangerous

So:

• Droge su opasne. = Drugs are dangerous.

Both this and Droga je opasna. can express a general idea, but the plural version matches English Drugs are dangerous more directly.