U ovoj rečenici glagol je u množini, iako je imenica u jednini.

It is in the locative case. The preposition u takes the locative when it means in in the sense of location, so u ovoj rečenici means in this sentence.

Here is the breakdown:

• ova → ovoj
• rečenica → rečenici

Both words change because they must match in case, number, and gender.

Because ovaj / ova / ovo must agree with the noun it describes. The noun rečenica is:

• feminine
• singular
• here in the locative case

So the correct form is ovoj. In other words, ovoj rečenici is a matching feminine singular locative phrase.

Because je is a clitic form of biti and clitics usually go very early in the clause. After conjunctions like iako, Croatian normally places je immediately after the conjunction, so iako je imenica u jednini is the natural order.

This is one of the places where Croatian word order differs from English. English would keep the noun is, but Croatian often puts the clitic earlier.

Literally, they mean in the plural and in the singular. Yes, this is a normal and common Croatian way to talk about grammatical number.

Both množina and jednina are nouns, and after u they appear in the locative:

• množina → množini
• jednina → jednini

So Croatian often uses a structure that is very close to English in the plural / in the singular.

Because iako means although / even though, and it introduces a subordinate clause. The sentence expresses contrast: the verb is plural, even though the noun is singular.

Ali means but, which is also a contrast word, but it works differently. You could build a different sentence with ali, but in this sentence iako is the right connector for the idea although X, Y.

Because iako introduces a subordinate clause, and Croatian normally separates that clause with a comma. So the comma marks the boundary between:

• the main clause: U ovoj rečenici glagol je u množini
• the subordinate clause: iako je imenica u jednini

This is standard Croatian punctuation.

Because Croatian does not have articles like the or a/an. Nouns appear without articles, and definiteness is understood from context.

So glagol can mean the verb here, and imenica can mean the noun, even though no separate word is used for the.

Yes. Croatian word order is more flexible than English word order, although not completely free. This sentence could be rearranged for emphasis or style, for example:

• Glagol je u ovoj rečenici u množini, iako je imenica u jednini.
• Iako je imenica u jednini, u ovoj rečenici glagol je u množini.

The meaning stays basically the same, but the focus changes slightly. The original version sounds natural and neutral.

The sentence can work without it if the context already makes clear which sentence you are talking about: Glagol je u množini, iako je imenica u jednini. But u ovoj rečenici makes the reference explicit: it tells you that this grammatical observation is about this sentence in particular.

So it is not required for grammar, but it is useful for clarity.