Treba mi opis bicikla.

Literally, it is something like A description of the bicycle is needed to me.

That sounds unnatural in English, but it helps explain the grammar:

• treba = is needed
• mi = to me
• opis bicikla = description of the bicycle / a bicycle description

So Croatian often expresses I need... with a structure that is closer to ...is needed to me than to English I need....

Mi is the dative form of ja.

Here it means to me:

• mi = to me
• ti = to you
• mu = to him
• joj = to her

So Treba mi opis bicikla literally uses the idea A description is needed to me.

This is a very common Croatian pattern.

Both are possible, but they are used a bit differently.

• mi is the normal short unstressed form
• meni is the full stressed form, usually for emphasis or contrast

So:

• Treba mi opis bicikla. = neutral, normal
• Meni treba opis bicikla. = I need a description of the bicycle, maybe not someone else

In most ordinary sentences, mi is the natural choice.

Opis is in the nominative singular.

In this sentence, the thing that is needed is opis. In this construction, Croatian treats the needed thing as the grammatical subject:

• Treba mi opis. = I need a description.
• Trebaju mi opisi. = I need descriptions.

Notice how the verb changes with number:

• singular: treba
• plural: trebaju

That shows that opis/opisi is functioning like the subject grammatically.

Because bicikla is genitive singular.

After the noun opis, Croatian often uses the genitive to mean of something:

• opis bicikla = description of the bicycle
• opis grada = description of the city
• opis osobe = description of the person

So bicikla is not the main noun here. It depends on opis and tells you what kind of description.

It can mean either, depending on context.

Croatian has no articles, so it does not directly mark a vs the the way English does.

So opis bicikla could be understood as:

• a description of a bicycle
• the description of the bicycle
• a bicycle description

The situation usually makes it clear which meaning is intended.

Because Croatian does not have articles like English.

So where English says:

• a bicycle
• the bicycle

Croatian simply says:

• bicikl
• bicikl

The listener understands from context whether it is definite or indefinite.

That is why opis bicikla can correspond to several English translations.

Yes, you may hear that, and many speakers do say it.

However, Treba mi opis bicikla is a very common and very safe pattern for learners when talking about needing a noun.

A useful way to think about the difference is:

• Treba mi opis bicikla. = A description of the bicycle is needed to me.
• Trebam opis bicikla. = I need a description of the bicycle.

Both can occur, but the first one is especially typical and often taught early because it is extremely common in everyday Croatian.

Yes, Croatian word order is fairly flexible.

The most neutral version here is:

• Treba mi opis bicikla.

But you could also hear:

• Opis bicikla mi treba.
• Meni treba opis bicikla.

The meaning stays similar, but the emphasis changes:

• Treba mi opis bicikla. = neutral
• Meni treba opis bicikla. = emphasizes me
• Opis bicikla mi treba. = emphasizes the description of the bicycle

One important point: short forms like mi often appear near the beginning of the sentence, in the usual clitic position.

Then the verb normally changes to plural too.

Compare:

• Treba mi opis bicikla. = I need a description of the bicycle.
• Trebaju mi opisi bicikala. = I need descriptions of bicycles.

So:

• opis → singular
• opisi → plural
• treba → singular
• trebaju → plural

This is a good clue that the noun being needed controls the verb form in this construction.

You can build many similar phrases.

The pattern is:

noun + genitive noun

Examples:

• opis bicikla = description of the bicycle
• slika grada = picture of the city
• vrata kuće = door of the house
• boja auta = color of the car

So this sentence is useful not just by itself, but also as a model for how Croatian often expresses of relationships.

Yes. A slightly more formal-sounding option is:

• Potreban mi je opis bicikla.

Literally, this is closer to A description of the bicycle is necessary/needed for me.

Compared with Treba mi opis bicikla, it can sound a bit more formal or deliberate. In everyday conversation, Treba mi opis bicikla is very natural.