Learn Hardcore Croatian: Trebao bih večeras ponoviti nove riječi umjesto da gledam seriju. - I should revise the new words tonight instead of watching a series. male speaker

As a normal verb trebati (to need):
- Trebam novac. = I need money.
- This usually expresses a more concrete need.
As a modal‑like verb meaning "should / ought to", it is most often used in the conditional:
- Trebao bih učiti. = I should study.
- Literally: "I would need to study."

So:

Trebao bih večeras ponoviti… = I should review tonight (advice, moral obligation, recommendation).
Trebam večeras ponoviti… = I need to review tonight (a stronger, more factual requirement).

Both are possible grammatically, but trebao bih matches the typical “I should” nuance better than trebam.

Why does it say trebao (masculine) when the subject is I? Shouldn’t it agree with first person?

In Croatian, the l‑participle (the form in trebao, radio, vidio, etc.) agrees in gender and number, not in person. Person is shown by the auxiliary (bih, si, smo, etc.), not by the participle.

Masculine speaker: Trebao bih…
Feminine speaker: Trebala bih…
Two or more males / mixed group: Trebali bismo…
Group of only females: Trebale bismo…

So if a woman says this sentence, it should be:

Trebala bih večeras ponoviti nove riječi umjesto da gledam seriju.

What exactly is the form trebao bih? Which tense or mood is this?

Trebao bih is Conditional I of trebati:

trebao – l‑participle (masculine singular)
bih – conditional form of biti (to be), 1st person singular

Together they form the conditional:

trebao bih – I would need (I should)
trebao bi – you (sg.) would need (you should)
trebali bismo – we would need (we should), etc.

It often corresponds to English should, ought to, or “would need to” depending on context.

Can I say bih trebao instead of trebao bih? Which word order is correct?

Both are possible, but the position of bih is restricted:

bih is a clitic, and clitics normally go in second position in the clause (after the first stressed word or phrase).
In your sentence, natural orders include:
- Večeras bih trebao ponoviti nove riječi…
- Ja bih večeras trebao ponoviti nove riječi…
- Trebao bih večeras ponoviti nove riječi… (here trebao is the first stressed word)

Putting bih first (Bih trebao…) is wrong.
So yes, Večeras bih trebao… is very common; Trebao bih večeras… (as in your example) is also fine.

Why is ponoviti used, and not ponavljati? What is the difference?

This is about aspect:

ponavljati – imperfective (to be repeating, to keep repeating, to do repeatedly / over some time)
ponoviti – perfective (to repeat once, to complete the act of reviewing)

In your sentence the focus is on doing the whole action at some point tonight, finishing that task:

Trebao bih večeras ponoviti nove riječi…
I should (once, successfully) review the new words tonight.

If you used ponavljati:

Trebao bih večeras ponavljati nove riječi…

it would sound more like “I should be (spending the evening) repeating the new words / doing repetition”, focusing on the ongoing process rather than a single completed study session.
Both are possible, but ponoviti is the more natural choice here.

Why is it nove riječi and not novih riječi?

The phrase nove riječi is in the accusative plural, because it is the direct object of ponoviti:

ponoviti što? → nove riječi

For feminine nouns like riječ:

Nominative plural: nove riječi (new words)
Accusative plural: nove riječi (same form as nominative)
Genitive plural: novih riječi

You would use novih riječi (genitive) in contexts like:

Imam novih riječi. – I have (some) new words.
Lista novih riječi. – A list of new words.

But as a direct object of ponoviti, accusative is needed, so nove riječi is correct.

Why is it umjesto da gledam, with gledam in the present, and not umjesto gledati?

The conjunction umjesto da (instead of doing X) is followed by a finite verb (a normal conjugated form), not by an infinitive:

umjesto da gledam – instead of (that I) watch
umjesto da gledam seriju – instead of watching a series

So:

umjesto gledati seriju – sounds wrong in standard Croatian.
Standard patterns are:
- umjesto da gledam seriju
- umjesto gledanja serije (using a verbal noun: instead of the watching of the series)

Even though gledam is present tense, with umjesto da it refers to the alternative action you could be doing tonight, similar to English “instead of watching”.

What case is seriju, and why does it end in -u?

Seriju is accusative singular feminine of serija (series/show):

Nominative singular: serija (subject form)
Accusative singular: seriju (direct object form)

You need the accusative because gledati (to watch) takes a direct object in the accusative:

gledam što? → seriju

So gledam seriju (I watch a series / the show) is the correct form.

Can I move večeras to another position in the sentence?

Yes, večeras (tonight) is an adverb of time and can move quite freely. All of these are grammatical, with slightly different emphasis:

Večeras bih trebao ponoviti nove riječi umjesto da gledam seriju.
Večeras trebam / trebao bih ponoviti nove riječi…
Trebao bih večeras ponoviti nove riječi… (your version)
Trebao bih ponoviti večeras nove riječi… (possible, but feels a bit marked/unusual)

The most natural options are usually:

Večeras bih trebao ponoviti…
Trebao bih večeras ponoviti…

Native speakers often put time adverbs near the beginning or just after the verb, but multiple orders are fine.

Why is there no ja (I) in the sentence? Is it optional?

Subject pronouns like ja (I), ti (you), on (he), etc., are normally dropped in Croatian because the verb form already shows the person:

Trebao bih… already clearly means I should.
Trebamo ponoviti… clearly means we should.

You can add ja for emphasis or contrast:

Ja bih trebao večeras ponoviti nove riječi, a ti možeš gledati seriju.
I should review the new words tonight, and you can watch the series.

But in a neutral sentence like yours, ja is normally omitted.

How would the sentence change if a woman says it, or if a group says it?

The auxiliary bih stays the same, but the participle trebao changes to agree with the subject’s gender and number:

One man:
- Trebao bih večeras ponoviti nove riječi umjesto da gledam seriju.
One woman:
- Trebala bih večeras ponoviti nove riječi umjesto da gledam seriju.
We (mixed group or all men):
- Trebali bismo večeras ponoviti nove riječi umjesto da gledamo seriju.
We (all women):
- Trebale bismo večeras ponoviti nove riječi umjesto da gledamo seriju.

Note that with we, the verb in the umjesto da clause also changes to 1st person plural:

umjesto da gledam → umjesto da gledamo.