Learn Hardcore Portuguese (Brazil): O contrato do novo apartamento foi assinado ontem pela minha irmã. - The contract for the new apartment was signed yesterday by my sister.

So o contrato do novo apartamento literally means the contract of the new apartment (often understood as the contract for the new apartment).

Why is it O contrato and not A contrato?

Because contrato is a masculine noun in Portuguese:

o contrato = the contract
um contrato = a contract

Gender is grammatical, so it doesn’t always match English intuition—you just learn it with the noun.

What is the grammar behind foi assinado?

foi assinado is the passive voice in the past:

foi = ser (to be), pretérito perfeito (simple past): was
assinado = past participle of assinar: signed

So it’s literally was signed.

Why is assinado masculine—shouldn’t it agree with minha irmã?

In the passive voice, the past participle agrees with the thing being signed, not the person who did it. Here, that’s o contrato (masculine singular), so:

o contrato foi assinado ✅
If the subject were feminine, it would change:
a carta foi assinada (the letter was signed) ✅

What does pela minha irmã mean, and why pela?

In Portuguese passive voice, the “doer” (agent) is often introduced by por = by.

pela is a contraction of por + a:

por = by
a = the (feminine singular)

So pela minha irmã = by my sister.
(If it were masculine: pelo meu irmão = por + o.)

Could I say this in active voice instead?

Yes, a very natural active version is:

Minha irmã assinou ontem o contrato do novo apartamento. Or with a common word order:
Minha irmã assinou o contrato do novo apartamento ontem.

Active voice is often more common in everyday speech, while passive can sound a bit more formal or “document-like.”

Why is ontem placed where it is?

Ontem (yesterday) is flexible in Portuguese. These are all possible and natural depending on emphasis:

O contrato ... foi assinado ontem pela minha irmã.
Ontem, o contrato ... foi assinado pela minha irmã.
O contrato ... foi assinado pela minha irmã ontem.

Putting ontem right after the verb phrase foi assinado is very common.

Does do novo apartamento mean the apartment is new, or the contract is new?

It describes apartamento (apartment), because novo is inside that noun phrase:

do novo apartamento = of the new apartment

If you wanted to say the contract is new, you’d say:

O novo contrato do apartamento... (The new contract of the apartment...)

Why is it novo apartamento and not apartamento novo?

Both can be correct, but they can suggest different nuance:

novo apartamento often means a new apartment (new to you / a different one, newly acquired)
apartamento novo often emphasizes that it is brand-new (not old, recently built/unused)

In real life, speakers may use both, but that nuance is a common guideline.

Why is it minha irmã and not meu irmã?

Possessives agree with the thing owned, not the owner. Since irmã is feminine:

minha irmã ✅ (my sister)
meu irmão ✅ (my brother)

So the possessive changes with irmã/irmão.

Do we always need o in O contrato?

In Portuguese, using the definite article (o/a/os/as) is very common when talking about a specific, known item (like a particular contract). You could omit it in some contexts (titles, notes, headlines), but in normal full sentences it’s typically:

O contrato foi assinado... ✅

Is foi assinado different from estava assinado?

Yes:

foi assinado = was signed (focus on the action/event of signing)
estava assinado = was signed / was already signed (focus on the state/condition at that time)

So foi assinado ontem clearly points to the signing happening yesterday.