Bicikl poput ovog je skup.

Poput means like, similar to, or of the kind/type of.

So Bicikl poput ovog je skup literally means something like:

• A bicycle like this one is expensive
• A bicycle similar to this one is expensive

In style, poput is a bit more formal or bookish than some other ways to say like.

Because poput requires the genitive case.

The basic demonstrative is:

• ovaj = this (masculine nominative)

But after poput, you need the genitive:

• ovog = of this / this one (masculine or neuter genitive singular)

So the structure is:

• poput ovog = like this one

You can think of an implied noun here:

• poput ovog bicikla = like this bicycle

In the sentence, bicikla is omitted because it is understood, so only ovog remains.

Here is the breakdown:

• Bicikl — nominative singular masculine
  
  • the subject: bicycle
• poput — a preposition that takes the genitive
• ovog — genitive singular masculine
  
  • of this / this one
• je — 3rd person singular of biti (to be)
• skup — masculine singular nominative adjective used predicatively
  
  • expensive

So the grammar is basically:

• [Subject in nominative] + [prepositional phrase] + [to be] + [predicate adjective]

Because skup is the normal short-form adjective used in this kind of predicate sentence.

Compare:

• Bicikl je skup. = The bicycle is expensive.

Here skup agrees with bicikl, which is masculine singular.

The form skupi can appear in other contexts, but in this sentence the natural standard form is skup.

So:

• bicikl = masculine singular
• predicate adjective = skup

If the noun changed, the adjective would change too:

• Knjiga je skupa. = The book is expensive.
• Auto je skupo. = The car is expensive.
• Bicikli su skupi. = The bicycles are expensive.

Je is the present-tense form of to be for he/she/it is.

In Croatian, unlike in some other languages, you normally do need this verb in a sentence like this:

• Bicikl je skup. = The bicycle is expensive.

So in your sentence:

• Bicikl poput ovog je skup.

the je cannot normally be omitted in standard Croatian.

Croatian has no articles like a or the, so bicikl by itself can mean either a bicycle or the bicycle, depending on context.

In this sentence, the most natural English translation is usually:

• A bicycle like this one is expensive.

But in another context, it could also be understood more definitely.

So learners should remember: Croatian often leaves definiteness to context instead of marking it with articles.

Yes, it is natural, but poput sounds a bit more formal or written.

Other common ways to express a similar idea are:

• Bicikl kao ovaj je skup. = A bicycle like this one is expensive.
• Ovakav bicikl je skup. = A bicycle like this is expensive / This kind of bicycle is expensive.

Nuance:

• poput ovog = somewhat more formal/literary
• kao ovaj = very common and conversational
• ovakav bicikl = more compact, often very natural

Ovog refers to an understood masculine noun, here bicikla.

So although only ovog is written, the full underlying idea is:

• poput ovog bicikla

That is why the form is masculine singular genitive.

Croatian often omits a repeated noun when it is obvious from context, just as English says this one instead of repeating the noun.

The given word order is natural and neutral:

• Bicikl poput ovog je skup.

Croatian word order is fairly flexible, but changing it can affect emphasis or sound less neutral.

For example:

• Skup je bicikl poput ovog.
  This puts more emphasis on expensive and may sound more marked.
• Bicikl je skup poput ovog.
  This changes the meaning and can become confusing, because now poput ovog may seem connected to skup rather than to bicikl.

So for learners, the original order is a good standard model.

Because Croatian often allows the noun to be omitted when it is clear.

So:

• poput ovog literally means like this one
• the full version would be poput ovog bicikla

This is very common with demonstratives and adjectives used pronominally.

English does something similar:

• I like this one
• Take the red one

Croatian can do the same:

• ovaj = this one
• ovog = of this one / this one in genitive contexts

Yes, ovoga is also possible.

In masculine/neuter genitive singular, Croatian often allows both:

• ovog
• ovoga

The shorter form ovog is very common, and the longer form ovoga can sound slightly fuller or more emphatic, depending on style and region.

So both of these are acceptable:

• Bicikl poput ovog je skup.
• Bicikl poput ovoga je skup.

The demonstrative and adjective would have to agree appropriately.

For example, with a feminine noun:

• Knjiga poput ove je skupa.
  A book like this one is expensive.

With a neuter noun:

• Računalo poput ovog je skupo.
  A computer like this one is expensive.

With a plural noun:

• Bicikli poput ovih su skupi.
  Bicycles like these are expensive.

This is a good reminder that Croatian agreement affects:

• demonstratives
• adjectives
• sometimes the verb, if the subject changes in number