Meu primeiro sintoma foi febre, então eu fiquei em casa.

Both are possible in Brazilian Portuguese:

• Meu primeiro sintoma (no article) is very common in everyday Brazilian Portuguese, especially with possessives.
• O meu primeiro sintoma is also correct; it can sound a bit more formal or emphatic, or it may reflect regional preference. So the sentence is natural as written, and adding o wouldn’t change the basic meaning.

Not naturally. The normal patterns are:

• Meu primeiro sintoma foi febre. (most natural)
• O meu primeiro sintoma foi febre. (also natural) You can move elements for emphasis in some contexts, but Meu sintoma primeiro sounds awkward. Ordinals like primeiro usually come right before the noun (primeiro sintoma) in this meaning.

No—Portuguese doesn’t say the symptom “had” a fever. Fever is the symptom. So you use:

• Meu primeiro sintoma foi febre. Or you can phrase it differently:
• O primeiro sintoma que eu tive foi febre. (“The first symptom I had was fever.”)

Yes:

• ..., então eu fiquei em casa. = very common, conversational, narrative flow.
• ..., por isso eu fiquei em casa. = a bit more explicit/clear “for that reason,” slightly more formal or written. Both are correct.

Portuguese often allows dropping the subject pronoun because the verb ending already shows the person:

• Então eu fiquei em casa. (explicit; very common in Brazilian Portuguese)
• Então fiquei em casa. (also correct; can sound a bit more concise) Brazilian Portuguese uses subject pronouns more often than European Portuguese, so including eu is very natural.

Both exist, but ficar is the most common verb for “to stay/remain” in many everyday situations:

• Fiquei em casa. = “I stayed home / I remained at home.” Estive em casa often sounds more like “I was at home (for a while)” and can feel less natural for the decision/result sense (“so I stayed home”). With então, fiquei fits better as a consequence.

In practice, fiquei em casa covers both ideas. Portuguese uses em casa (“at home”) where English often just says “home.”
If you want to stress not going out, you could also say:

• Fiquei em casa (sem sair). = “I stayed home (without going out).”

Because casa without an article often means “home” in a general/home sense:

• em casa = “at home” na casa (= em + a) usually means “in/at the house” as a specific place/building (often not necessarily your home), or when you’re specifying which house:
• na casa da minha mãe = “at my mother’s house”

Yes. Both are possible:

• foi febre is very common when naming a condition/symptom in a general way.
• foi a febre can sound a bit more specific/definite (like “it was the fever”), but in many contexts it’s just a stylistic choice. In everyday Brazilian Portuguese, foi febre is often the more straightforward option.

It’s recommended because you have two independent clauses:

• Meu primeiro sintoma foi febre, então eu fiquei em casa. In informal writing (texts, messages), people may omit it, but in standard writing the comma helps readability.

Yes, depending on region and style:

• ..., daí eu fiquei em casa. is common in speech and informal writing in Brazil, often meaning “then/so.” It’s more casual and can be more regional. Então is more neutral nationwide.

Sintoma matches “symptom” closely (especially medical context).
Sinal can be “sign” (an observable sign, clue), and it’s used too, but it’s not always interchangeable. In health contexts:

• sintoma = what the person feels/reports (e.g., fever, pain, nausea)
• sinal = what can be observed/measured (e.g., high temperature, rash) In everyday conversation people may mix them, but sintoma is the safest translation for “symptom.”

Common options:

• Eu tive febre. = “I had a fever.”
• Eu fiquei com febre. = “I came down with a fever / I got a fever.”
• Comecei com febre. = “I started with a fever (as the first symptom).” Each is natural; fiquei com febre often emphasizes the onset.