U drugoj rečenici subjekt nije izrečen, ali se predikat lako prepoznaje.

Here it means second.

The word drugi / druga / drugo can mean either second or other/another, so context matters. In U drugoj rečenici, the natural reading is In the second sentence.

So:

• druga rečenica = the second sentence
• druga knjiga can mean the second book or another book, depending on context

Because u can take different cases depending on meaning.

Here, u drugoj rečenici uses the locative, because it means in the second sentence as a location or context.

• u + locative = in, within
• u + accusative = into, to with movement

So:

• u drugoj rečenici = in the second sentence
• u drugu rečenicu = into the second sentence

Also notice the forms:

• druga → drugoj
• rečenica → rečenici

Both change because they are in the feminine singular locative.

Izrečen is a past passive participle used predicatively with nije.

So subjekt nije izrečen literally means the subject is not expressed / not stated.

A few important points:

• subjekt is masculine singular
• therefore the participle is also masculine singular: izrečen
• if the noun were feminine, the form would change: nije izrečena
• if it were neuter: nije izrečeno

Compare:

• subjekt nije izrečen
• tema nije izrečena
• pravilo nije izrečeno

In this sentence, izrečen means something like explicitly stated in words.

Yes. Croatian very often allows the subject to be omitted when it is clear from the verb form or context.

For example:

• Pišem. = I am writing.
• Idemo. = We are going.

English usually needs the subject pronoun, but Croatian often does not.

So when this sentence says subjekt nije izrečen, it is talking about a very common feature of Croatian: the subject may be understood without being explicitly stated.

Here se is not really reflexive in the English sense of it recognizes itself.

In this sentence, se prepoznaje is a passive-like / impersonal construction. It means:

• is easily recognized
• or can easily be recognized

So:

• predikat se lako prepoznaje = the predicate is easily recognized

This is very common in Croatian. The speaker does not say who recognizes it; the focus is on the fact that it is recognizable.

Yes, in grammatical terms, predikat functions as the subject of se prepoznaje.

That may feel strange at first, because the verb prepoznati is normally transitive:

• Prepoznajemo predikat. = We recognize the predicate.

But with se, Croatian often shifts to a passive-like meaning:

• Predikat se prepoznaje. = The predicate is recognized / can be recognized.

So the focus moves from who recognizes it to the predicate itself.

Because se is a clitic, and Croatian clitics usually stand near the beginning of the clause.

That is why ali se predikat lako prepoznaje sounds natural and standard.

A learner should remember this general idea:

• words like se, je, sam, si, ga, mu often do not stand freely
• they usually appear in an early position in the clause

So ali se... is the expected pattern here.

Yes, you may hear or see that order too, especially if predikat is being emphasized. But the more neutral textbook-style order here is ali se predikat lako prepoznaje.

Croatian word order is flexible, but it is not random. Changing the order can change emphasis, rhythm, or style.

So:

• ali se predikat lako prepoznaje = neutral
• ali predikat se lako prepoznaje = possible, with a bit more focus on predikat

Because lako is an adverb, and it modifies the verb prepoznaje.

It tells us how the predicate is recognized:

• lako prepoznaje = is recognized easily

By contrast, lak is an adjective:

• lak zadatak = an easy task

So here you need the adverb, not the adjective.

Because prepoznaje is the imperfective present form, and that is the normal choice for a general statement like this.

The sentence is expressing something generally true:

• the predicate is easy to recognize

So Croatian uses the imperfective verb prepoznavati / prepoznavati? Actually in standard usage here, prepoznaje functions as the ordinary present form used for a general process/state meaning is recognized.

For a learner, the key point is this:

• prepoznaje works well for general present meaning
• prepozna is from a perfective verb and usually does not work as a simple present in the same way

So lako se prepoznaje is the natural form for is easily recognized.