Podcrtana riječ nije imenica, nego glagol.

Because riječ is a feminine singular noun, and Croatian adjectives must agree with the noun they describe in gender, number, and case.

So:

• podcrtan = masculine
• podcrtana = feminine
• podcrtano = neuter

Here we have riječ, so the correct form is podcrtana riječ.

A useful thing to notice: riječ is feminine even though it does not end in -a.

Literally, it means underlined word.

Croatian normally puts adjectives before the noun, just like English does in phrases such as underlined word or big house.

So:

• podcrtana riječ = underlined word
• velika kuća = big house

This is the normal, neutral word order.

Nije means is not.

It is the 3rd person singular present form of biti (to be) in the negative.

Compare:

• je = is
• nije = is not

This is a special pattern. With most Croatian verbs, ne is written separately:

• ne govorim = I do not speak
• ne znam = I do not know

But with biti, the negative present forms are written as one word:

• nisam
• nisi
• nije
• nismo
• niste
• nisu

Because nego is the usual word after a negated statement when you replace one idea with the correct one.

The pattern is:

nije X, nego Y = not X, but Y

So:

• nije imenica, nego glagol = it is not a noun, but a verb

Ali also means but, but it is more general. In this sentence, nego is the natural choice because the speaker is correcting the first label.

Compare:

• Nije imenica, nego glagol. = It’s not a noun, but a verb.
• Htio sam doći, ali nisam mogao. = I wanted to come, but I couldn’t.

They are in the nominative singular.

In this sentence:

• Podcrtana riječ is the subject
• imenica and glagol are predicate nouns after biti

In simple identification sentences like this, Croatian normally uses the nominative:

• Ovo je knjiga.
• On je učitelj.
• To nije problem.

So imenica and glagol are just their normal dictionary forms here.

Because Croatian does not have articles like English a/an/the.

So:

• riječ can mean a word or the word
• glagol can mean a verb or the verb

The exact meaning is understood from context. In this sentence, English naturally translates it as the underlined word is not a noun, but a verb, because a specific word is being discussed.

Yes, Croatian word order is more flexible than English word order, because Croatian uses endings to show grammatical relationships.

The sentence as given is the most neutral and natural version:

Podcrtana riječ nije imenica, nego glagol.

But if the context is already clear, Croatian can drop the subject phrase and simply say:

Nije imenica, nego glagol.

That still means It’s not a noun, but a verb.

So the original sentence is the full, clear version, especially in a classroom or grammar explanation.

Because the subject is already stated: Podcrtana riječ.

English often needs a subject pronoun, but Croatian does not add an extra it when the noun is already there.

So Croatian says:

• Podcrtana riječ nije imenica, nego glagol.

not something like:

• Podcrtana riječ ona nije...

If you were continuing from earlier context and did not want to repeat podcrtana riječ, you could use a pronoun:

• Ona nije imenica, nego glagol.

Here ona is used because riječ is feminine.

In this sentence, yes: riječ clearly means word.

But in other contexts, riječ can have a few related meanings, such as:

• word
• speech
• what someone said
• promise in some expressions

For example:

• To je duga riječ. = That is a long word.
• Daj mi riječ. = Give me your word.

In a grammar/classroom sentence like this one, riječ definitely means word.