Eu fiquei em casa ontem à noite porque estava cansado.

In Brazilian Portuguese, ficar em casa is the most natural way to say to stay at home (remain somewhere).

• Eu fiquei em casa = I stayed at home (I remained at home).
• Eu estive em casa usually means I was at home (at some point), often more like “I happened to be there / I was there,” not emphasizing “staying in.”
• Eu estava em casa describes a state/background (“I was at home”), but it doesn’t highlight the decision/action of staying the way fiquei does.

Fiquei is the pretérito perfeito (simple past). It presents the action as completed and tied to a specific past time: ontem à noite (last night). It suggests that you did stay home during that time period.

Estava is pretérito imperfeito (imperfect) and is commonly used to give a reason/background condition in the past:

• ...porque estava cansado = because I was tired (as a continuing state at that time).

Using estive cansado is unusual in Portuguese; tiredness is typically described with estar in the imperfect in this kind of explanation. Fui cansado is not used to mean “was tired.”

No. Portuguese usually omits repeated subject pronouns when the subject is clear from context and verb form:

• Eu fiquei... porque estava cansado. (natural)
  You can repeat it for emphasis or contrast, but it’s often unnecessary:
• ...porque eu estava cansado. (more explicit, slightly heavier)

Em casa is an idiomatic expression meaning at home (home as a concept, not a specific building).

• em casa = at home
• na casa ( = em + a) tends to mean in/at the house (a specific house), often with extra context: na casa do João (at João’s house).

Ontem à noite means last night (literally “yesterday at night”).
Word order is flexible:

• Eu fiquei em casa ontem à noite... (very common)
• Ontem à noite, eu fiquei em casa... (also common, more emphasis on time)

À is the contraction of a + a (preposition a + feminine article a):

• a (to/at) + a noite (the night) → à noite
  It’s written with a grave accent (crase) to show the contraction.

Yes, often. In Brazil:

• à noite commonly means in the evening / at night in a specific-time sense.
• de noite can be similar and is also common, sometimes with a more general “at night (in general)” feel.
  In this sentence, ontem à noite is the standard, very natural choice.

Here, porque (one word) means because, introducing a reason:

• ...porque estava cansado.

You use por que (two words) mainly in questions (meaning “why” / “for what reason”):

• Por que você ficou em casa? = Why did you stay at home?

Adjectives agree with the speaker/person being described.

• Male speaker: cansado
• Female speaker: cansada

So a woman would typically say: ...porque estava cansada.

Not necessarily. Ficar em casa is neutral: “to stay home.”
If you mean “got stuck,” you’d normally add context, for example:

• Eu fiquei preso em casa = I got stuck at home / I was trapped at home.
• Eu tive que ficar em casa = I had to stay home.

Typical Brazilian pronunciations (approximate):

• fiquei: sounds like fi-KAY (the que here has a hard k sound: /fiˈkej/)
• porque: often like por-KEH (/poʁˈke/), with a “guttural” r in many accents.