A criança ficou doente ontem.

Here a means the, not a/an.

In Portuguese, a is the feminine singular definite article:

• a criança = the child

The indefinite article a/an would be uma:

• uma criança = a child

So:

• A criança ficou doente ontem. = The child got sick yesterday.
• Uma criança ficou doente ontem. = A child got sick yesterday.

Yes. Criança can refer to either a boy or a girl.

It is a grammatically feminine noun, so it takes:

• a as its article
• feminine agreement with words that change for gender

But its real-life meaning is still child, regardless of sex.

For example:

• A criança está cansada. = The child is tired.

Even if the child is a boy, many speakers still say a criança because the noun itself is feminine.

If you want to make the sex clear, you can use:

• o menino = the boy
• a menina = the girl

Ficou is the preterite form of ficar, third person singular.

That means it matches:

• ele ficou
• ela ficou
• a criança ficou

The preterite is commonly used for a completed event in the past, especially with a time word like ontem (yesterday).

So ficou tells you that the change happened at a specific past moment.

Because ficou doente emphasizes a change of state.

• ficou doente = got sick / became sick
• estava doente = was sick

So the difference is:

• A criança ficou doente ontem.
  The child became sick yesterday.

• A criança estava doente ontem.
  The child was sick yesterday.

In other words, ficar + adjective often means to become something.

Not in this sentence.

The verb ficar has several meanings depending on context, including:

• to stay
• to remain
• to become

With an adjective like doente, it usually means to become:

• ficar doente = to get sick
• ficar feliz = to become happy
• ficar triste = to become sad

So here ficou doente does not mean stayed sick. It means got sick.

Because doente is an adjective with the same form for masculine and feminine.

So you say:

• o menino doente
• a menina doente
• a criança doente

Unlike adjectives such as:

• cansado / cansada
• bonito / bonita

Doente does not change form for gender.

Yes. Doente is an adjective, and in this sentence it works as a predicate adjective after the verb ficar.

Structure:

• A criança = subject
• ficou = verb
• doente = adjective describing the subject
• ontem = time expression

So doente describes the state the child entered.

Yes. A criança adoeceu ontem is also correct.

• adoecer = to become ill / to fall sick
• adoeceu = got sick / fell ill

So these are very close:

• A criança ficou doente ontem.
• A criança adoeceu ontem.

A small nuance:

• ficou doente is very common in everyday speech
• adoeceu can sound a bit more direct or slightly more formal, depending on context

Both are natural in Brazilian Portuguese.

Yes. Ontem is flexible.

These are all possible:

• A criança ficou doente ontem.
• Ontem, a criança ficou doente.

Both are natural. The second version puts a little more focus on yesterday.

The version with ontem at the end is very common and neutral.

In normal standard speech, you would usually keep a:

• A criança ficou doente ontem.

Without the article:

• Criança ficou doente ontem

this sounds more like:

• a headline
• a note
• very telegraphic speech

Portuguese often uses articles where English may not, so learners should generally keep them unless there is a specific reason not to.

In Brazilian Portuguese, criança is roughly pronounced like:

kree-AHN-sah

A slightly more accurate IPA version is: /kɾiˈɐ̃sɐ/

A few helpful points:

• cri sounds like kree
• ç sounds like s
• ã is a nasal vowel, so the sound is nasalized
• the stress is on the second syllable: -AN-

So the word is not pronounced with a hard k sound at the end or like English child. The nasal vowel is one of the trickier parts for English speakers.

The basic order is very normal:

• Subject + verb + complement + time expression

So:

• A criança = subject
• ficou = verb
• doente = complement
• ontem = time

This order is natural and neutral in Brazilian Portuguese.

Portuguese word order is somewhat flexible, but this sentence is a very standard way to say it. For a learner, it is a good model to follow.