A ameaça desapareceu.

Because a is the definite article meaning the for a feminine singular noun.

• a ameaça = the threat
• uma ameaça = a threat

In Portuguese, articles are used very often, more often than in English in some contexts.

Yes, ameaça is a feminine singular noun.

You can see that from:

• the article a
• the noun ending -a, which often (though not always) signals a feminine noun

So:

• a ameaça = feminine singular
• as ameaças = feminine plural

The ç is called c cedilha. It makes the c sound like s.

So ameaça is pronounced with an s sound, not a k sound.

Compare:

• ca = usually a k sound
• ça = an s sound

That is why ameaça sounds roughly like ah-meh-AH-suh in a learner-friendly approximation.

Desapareceu is the pretérito perfeito of desaparecer.

In this sentence, it means something like:

• disappeared
• has disappeared, depending on context

It refers to a completed event in the past.

The infinitive is desaparecer = to disappear.

Desapareceu is the 3rd person singular form in the pretérito perfeito:

• eu desapareci
• tu desapareceste
• ele/ela/você desapareceu
• nós desaparecemos
• vós desaparecestes
• eles/elas/vocês desapareceram

In A ameaça desapareceu, the subject is a ameaça, so the verb is in 3rd person singular.

Because the subject, a ameaça, is singular.

Portuguese verbs agree with the subject:

• A ameaça desapareceu. = singular subject + singular verb
• As ameaças desapareceram. = plural subject + plural verb

So the ending of the verb changes depending on who or what is doing the action.

Because Portuguese usually does not need an explicit subject pronoun when the subject is already stated.

English often says:

• The threat disappeared.

Portuguese says:

• A ameaça desapareceu.

There is no need to add a separate word for it, because a ameaça is already the subject.

Yes, you can, although the most neutral order is:

• A ameaça desapareceu.

Portuguese allows some flexibility in word order.
Desapareceu a ameaça can sound more literary, emphatic, or context-driven, as if highlighting the disappearance itself.

So both are possible, but A ameaça desapareceu is the straightforward default.

Not always, but in this sentence it is natural and normal.

• A ameaça desapareceu. = The threat disappeared.
• Uma ameaça desapareceu. = A threat disappeared.

Without an article, Portuguese usually sounds less natural unless you are speaking in a special style, such as headlines, labels, or very general statements.

A simple learner-friendly approximation is:

duh-zah-puh-ruh-SEU

A few useful points:

• the stress falls on the last part: -ceu
• the ending eu in European Portuguese has a sound that may be unfamiliar to English speakers
• the s in desapareceu sounds like z here because it comes between vowels

You do not need perfect pronunciation at first; the most important thing is recognising the word and stressing the final syllable.

Yes, very closely. It is an intransitive verb, so it normally does not take a direct object.

That means:

• A ameaça desapareceu. = correct
• not something like A ameaça desapareceu-o.

The subject itself disappears.

Yes. The Portuguese pretérito perfeito often corresponds to the English simple past, but in many contexts English may naturally translate it as has disappeared.

So depending on context, A ameaça desapareceu could be understood as:

• The threat disappeared
• The threat has disappeared

The exact English choice depends on the surrounding context, not on a different Portuguese form here.

The plural would be:

As ameaças desapareceram.

Changes:

• a → as
• ameaça → ameaças
• desapareceu → desapareceram

This is a good example of how both the noun phrase and the verb change in Portuguese when the subject becomes plural.