O peixe morreu ontem.

In this sentence, o means the, so o peixe = the fish.

In Portuguese, singular count nouns usually need an article or some other determiner. So:

• O peixe morreu ontem. = The fish died yesterday.

Just Peixe morreu ontem would sound incomplete or unnatural in normal Portuguese.

Peixe is a masculine noun in Portuguese, so it takes the masculine article o.

• o peixe = the fish

This is grammatical gender, not biological sex. Even if the fish were female in real life, the noun peixe is still grammatically masculine unless you specifically say something like a fêmea do peixe or use another word to clarify sex.

Also, nouns ending in -e can be either masculine or feminine, so you often just have to learn the gender with the word.

Morreu is the third-person singular preterite form of the verb morrer (to die).

So:

• morrer = to die
• ele/ela/você morreu = he/she/you died

In this sentence, the subject is o peixe, which is third-person singular, so the verb must be morreu.

Morreu is used because the sentence describes a completed event in the past.

• morreu = died / has died, depending on context
• morria would suggest was dying, used to die, or some ongoing/background idea, which does not fit here
• tem morrido usually means has been dying or has died repeatedly / in recent periods, which also does not fit this simple one-time event

Because ontem means yesterday, the simple completed past (preterite) is the natural choice:

• O peixe morreu ontem.

No, that would not be correct.

There is an important difference:

• morreu = died (the event happened)
• está morto = is dead (the resulting state)

So:

• O peixe morreu ontem. = The fish died yesterday.
• O peixe está morto. = The fish is dead.

If you want to say when the death happened, Portuguese normally uses morreu, not está morto with ontem.

Yes, ontem can move, but putting it at the end is very natural and common.

These are all possible:

• O peixe morreu ontem.
• Ontem, o peixe morreu.
• O peixe ontem morreu. — possible, but less neutral and less common in everyday speech

The most neutral word order is:

• subject + verb + time expression

So O peixe morreu ontem sounds very normal.

In Brazilian Portuguese, peixe is pronounced roughly like PAY-shi.

More closely:

• pei sounds like pay
• x here sounds like sh
• the final e in Brazilian Portuguese is often pronounced like a light i sound

So:

• peixe ≈ PAY-shi

It is not pronounced like English peeks or payks.

In Brazilian Portuguese, morreu is pronounced roughly mo-HEW, with the stress on the last syllable.

A few details:

• the rr in Brazilian Portuguese is usually a throaty h-like sound
• eu here sounds like a diphthong, somewhat like ehw / eoo

So an English-friendly approximation is:

• morreu ≈ mo-HEW

The real Portuguese sound is a bit different from English, especially the rr.

Ontem is pronounced roughly ON-teng or ON-teyn, but with nasal vowels.

Important points:

• the on is nasal
• the final em is also nasal

An English approximation can help, but the true Portuguese pronunciation involves airflow through the nose, which English speakers often need practice with.

Here it is singular: o peixe = the fish.

In English, fish can be singular or plural, so the form does not always tell you much. In Portuguese, the article helps:

• o peixe = the fish
• os peixes = the fish / the fishes

Also, the verb confirms it:

• morreu = singular
• morreram = plural

So:

• O peixe morreu ontem. = The fish died yesterday.
• Os peixes morreram ontem. = The fish died yesterday. / The fishes died yesterday.

Yes, you can, but it changes the feeling of the sentence.

Portuguese often allows the subject to be omitted when it is clear from context. So:

• Morreu ontem. = It/He/She died yesterday.

However, by itself, this sentence does not tell you who or what died. In your original sentence, o peixe makes that explicit.

So:

• O peixe morreu ontem. = clear and complete
• Morreu ontem. = natural only if the listener already knows what you are talking about

Because Portuguese usually expresses the past directly through the verb ending.

English often uses:

• did die in some contexts
• or just died

Portuguese does not need a separate helping verb here. The past meaning is built into morreu.

So:

• morreu already means died

That is why O peixe morreu ontem is complete without any extra word like did.

Grammatically, it is possible, but it means something different:

• O peixe morreu ontem. = The fish died yesterday.
• Um peixe morreu ontem. = A fish died yesterday.

So the choice between o and um depends on whether you mean a specific fish already known in the conversation (the fish) or just one fish, not previously identified (a fish).

It works in both. O peixe morreu ontem is completely normal, natural Portuguese.

It is:

• grammatically standard
• natural in conversation
• acceptable in writing

So this is a very useful model sentence for everyday Brazilian Portuguese.