Learn Hardcore Croatian: Naš protivnik je večeras jak, ali se ne bojimo poraza. - Our opponent is strong tonight, but we are not afraid of defeat.

The full verb is bojati se, not just bojati.
Without se, bojati means something else (mostly “to dye/colour/paint” in modern usage).
So bojimo se literally looks like “we fear ourselves”, but idiomatically it just means “we are afraid”.

In English you don’t say “we fear ourselves of defeat”, you just say “we are afraid of defeat”. Croatian keeps the reflexive pronoun, so you must include se whenever you use bojati se.

Why is se placed after ali and before ne bojimo? Why not ali ne bojimo se or ne se bojimo?

Two separate rules are at play:

Clitic position
Se is a clitic (an unstressed little word). In Croatian, clitics normally stand in “second position” in a clause, right after the first stressed word.
- The new clause starts after ali, so se naturally comes right after it:
  - Ali se ne bojimo poraza.
Other possible orders
- Ali se ne bojimo poraza. – fully standard, very natural.
- Ali ne bojimo se poraza. – also possible and used, especially in speech; the negative ne sticks tightly to bojimo, and se follows the verb.
- *Ne se bojimo poraza. – incorrect; se cannot come directly after ne at the start like this.
- *Se ne bojimo poraza. – incorrect as a full sentence, because a clitic cannot stand first.

So the given word order follows the usual “second position” pattern for se after ali.

Why is it poraza and not poraz? What case is that?

Poraza is the genitive singular of poraz (“defeat”).

The verb bojati se always takes its object in the genitive case:

Bojimo se poraza. – We are afraid of defeat.
Bojim se psa. – I am afraid of the dog. (genitive singular: psa)
Boje se rata. – They are afraid of war. (genitive singular: rata)

So it is not accusative poraz, but genitive poraza, required by the verb bojati se.

Is poraza genitive because the verb is negative (ne bojimo se) or would it be the same in a positive sentence?

It would be the same in a positive sentence. The genitive comes from the verb bojati se, not from the negation.

Bojimo se poraza. – We are afraid of defeat. (affirmative)
Ne bojimo se poraza. – We are not afraid of defeat. (negative)

In both cases the object is in genitive (poraza). The negative ne does not change the case here; it only adds “not”.

Why isn’t there a word for we in ali se ne bojimo poraza?

Croatian usually drops subject pronouns because the verb ending already shows the person and number.

Bojim se – I am afraid (1st person singular, “I” is clear from -im)
Bojimo se – We are afraid (1st person plural, “we” is clear from -imo)

So:

Ali se ne bojimo poraza. = But (we) are not afraid of defeat.

You can say Ali mi se ne bojimo poraza., but mi is only used for emphasis (e.g. “But we are not afraid of defeat”, in contrast to others).

Why is it jak and not jaki or jakog?

Jak is an adjective meaning “strong”. Here it is used as a predicate after je (“is”), so it must:

Agree in gender and number with the subject protivnik (masculine singular), and
Be in the nominative case, because predicate adjectives take nominative.

So we get:

Naš protivnik je … jak. – Our opponent is strong.
- protivnik – masculine singular nominative
- jak – masculine singular nominative (predicate)

Other forms would be used in different roles:

jakog protivnika – genitive/accusative singular (“a strong opponent” as an object)
jaki protivnik – adjective before the noun (“the strong opponent”)

As soon as the adjective goes after je and describes the subject, you use the nominative form: jak.

Could I leave out je and just say Naš protivnik večeras jak like in English “Our opponent tonight strong”?

In normal Croatian, you cannot omit je here. You need the verb biti (“to be”) in this kind of sentence:

Naš protivnik je večeras jak. – correct
*Naš protivnik večeras jak. – wrong in standard Croatian (sounds like a newspaper headline or note, not a full sentence)

Croatian does not drop je in regular sentences the way English can in short phrases or headlines.

Can I move večeras to a different place in the sentence?

Yes. Večeras (“tonight / this evening”) is an adverb and can move around quite freely. These are all correct:

Naš protivnik je večeras jak. – neutral; common order.
Večeras je naš protivnik jak. – emphasizes tonight: “Tonight, our opponent is strong.”
Naš protivnik večeras je jak. – also possible; slight emphasis on this evening as the time frame.
Naš je protivnik večeras jak. – emphasizes naš (“our”); the clitic je moves after the first stressed word Naš.

The basic meaning stays the same; only the focus/emphasis changes a bit with word order.

Does večeras mean “this evening” or “tonight”? Is it different from noćas?

Večeras literally means “in the evening (today)” and in practice covers what English splits into “this evening” and “tonight” (the early part of the night).

Večeras gledamo utakmicu. – We’re watching the match this evening / tonight.

Noćas means “during the night (tonight)” and usually refers to the later, night-time hours:

Noćas nisam spavao. – I didn’t sleep last night / during the night.

In your sentence, večeras is the natural choice, because a match or competition usually happens in the evening, not in the middle of the night.

What gender and number are naš and protivnik, and how do they agree?

Protivnik (“opponent”) here is masculine singular, nominative.
Naš (“our”) is a possessive pronoun that has to agree with the noun it modifies. So it also appears as masculine singular nominative.

That’s why we have:

naš protivnik – our (male) opponent

If the opponent is female, you change both the noun and the possessive:

naša protivnica je večeras jaka.
- naša – feminine singular nominative
- protivnica – feminine singular nominative
- jaka – feminine singular nominative predicate adjective

For plural:

naši protivnici su večeras jaki. – Our opponents are strong tonight.

Why is naš before protivnik? Could I say protivnik naš?

In standard Croatian, possessive pronouns almost always come before the noun:

naš protivnik, tvoja sestra, njihov prijatelj

Putting the possessive after the noun (protivnik naš) is unusual and would sound:

poetic or stylistic, or
dialectal / very marked in everyday speech.

So in normal conversation you say naš protivnik, not protivnik naš.

Is there any difference between ali, no, and nego here? Could I replace ali?

In this sentence:

Ali se ne bojimo poraza.

You could also say:

No se ne bojimo poraza.

Both ali and no mean roughly “but”, and here they are almost interchangeable. No can sound a bit more literary or formal, depending on context.

Nego, however, is different. It usually means “but rather / but instead” and is used after a negation to introduce a contrastive alternative:

Ne bojimo se poraza, nego ga priželjkujemo.
We are not afraid of defeat, but we actually wish for it.

So for your sentence you use ali or no, not nego.

How would I say the affirmative version: “We are afraid of defeat”?

You simply drop ne:

Bojimo se poraza. – We are afraid of defeat.

The structure stays the same:

verb bojimo (1st person plural)
reflexive se
genitive object poraza

How would I say “Our opponent will be strong tonight”?

Use the future tense of biti (“to be”):

Naš protivnik će večeras biti jak.

Breakdown:

Naš protivnik – our opponent (subject)
će – future auxiliary (clitic)
večeras – tonight
biti – infinitive “to be”
jak – strong

You can also change the word order a bit:

Večeras će naš protivnik biti jak.
Naš će protivnik večeras biti jak.

In each case the little future particle će behaves like a clitic and usually goes in “second position” in its clause.