O fim de semana passado foi ótimo.

The gender of the phrase is determined by the main noun fim (end), not by semana (week).

• fim is masculine → o fim
• semana is feminine → a semana

In the expression o fim de semana, the structure is literally “the end of (the) week”.
So the head noun is fim (masculine), and de semana is just a complement.
That’s why you say o fim de semana, not a fim de semana.

Yes. de here works very much like of in English.

• fim de semana = end of (the) week → “weekend”

This de links two nouns, where the second defines or specifies the first:

• fim de semana – end of week
• garrafa de água – bottle of water
• copo de vinho – glass of wine

Both forms exist, but the current standard in Portugal (after the spelling reform, Acordo Ortográfico) is fim de semana without hyphens.

• Older/less current: fim-de-semana
• Modern standard: fim de semana

You will still see fim-de-semana in older texts or from people who don’t follow the reform, but if you’re learning contemporary European Portuguese, use fim de semana.

Approximate European Portuguese pronunciation:

• IPA: [u ˈfĩ dɨ sɨˈmɐ.nɐ pɐˈsaðu fwe ˈɔ.ti.mu]
• Rough English-like guide: “oo feeng dɨ sɨ-MA-na pa-SAH-du fwee Ó-ti-mu”

Notes:

• Final m in fim is nasal: like “feeng” but shorter.
• de is reduced to dɨ, a very short, weak vowel.
• semana is stressed on -MA-: se-MA-na.
• ótimo is stressed on the first syllable: Ó-ti-mo, with an open ó sound.

In Portuguese, most adjectives normally come after the noun:

• fim de semana passado – literally “weekend past”
• carro novo – new car
• casa grande – big house

Time expressions with passado usually follow this pattern:

• o mês passado – last month
• o ano passado – last year
• a semana passada – last week

So fim de semana passado is the standard word order.

Yes, you can say último fim de semana, and it is correct in European Portuguese.

• fim de semana passado – last weekend
• último fim de semana – literally “last weekend” as well

In everyday speech:

• fim de semana passado is very common and sounds completely natural.
• último fim de semana is also used; it can sometimes sound a bit more formal or written, but the difference is small.

For most situations, you can treat them as synonyms.

foi is the simple past (pretérito perfeito) of ser, and it presents the weekend as a completed event in the past:

• O fim de semana passado foi ótimo. – It was great (as a finished whole).

era (imperfect of ser) is used for ongoing, repeated, or background descriptions in the past:

• Quando eu era criança, os fins de semana eram ótimos.
  “When I was a child, weekends were great (in general, habitually).”

estava (imperfect of estar) is used for temporary states or conditions, often in progress:

• Eu estava cansado no fim de semana passado.
  “I was tired last weekend.”

So foi is the right choice to describe how that specific, completed weekend was overall.

In Portuguese, we normally use ser with adjectives to classify or evaluate whole events:

• O filme foi ótimo. – The film was great.
• A viagem foi horrível. – The trip was horrible.
• O fim de semana passado foi ótimo. – The past weekend was great.

estar is used more for temporary states or conditions of people or things at a certain moment:

• Eu estive ótimo na apresentação. – I was great in the presentation.
• Ele esteve doente. – He was ill (for a while).

You might occasionally hear something like “O concerto esteve ótimo”, but “foi ótimo” is much more standard when evaluating an event.

Yes, ótimo is a regular adjective and agrees with the noun:

• Masculine singular: ótimo
• Feminine singular: ótima
• Masculine plural: ótimos
• Feminine plural: ótimas

Examples:

• O fim de semana foi ótimo. – masculine singular
• A festa foi ótima. – feminine singular
• Os fins de semana foram ótimos. – masculine plural
• As férias foram ótimas. – feminine plural

The plural is fins de semana (both fim and the article change; semana stays the same):

• Singular: o fim de semana
• Plural: os fins de semana

To make the sentence plural:

• Os fins de semana passados foram ótimos.
  “The past weekends were great.”

Here you also:

• change o → os
• fim → fins
• foi → foram (3rd person plural of ser)
• ótimo → ótimos (to agree with fins de semana)

You can say O fim de semana foi ótimo, and in context people will often understand you mean the most recent weekend, especially if you’re talking on Monday or Tuesday.

However, to be clear and explicit, Portuguese normally uses words like passado or último with time expressions:

• O fim de semana passado foi ótimo.
• A semana passada foi complicada.

So:

• With clear context: O fim de semana foi ótimo. is fine.
• If you want to be precise or the context isn’t obvious: use fim de semana passado.

Here passado is an adjective meaning “past” or “last”.

It comes from the past participle of the verb passar (“to pass”), but in this sentence it behaves like an adjective describing a noun of time:

• o ano passado – the past year / last year
• a semana passada – the past week / last week
• o fim de semana passado – the past weekend / last weekend

So in O fim de semana passado foi ótimo, passado is not the verb; the verb is foi (from ser).

No, O passado fim de semana is not natural Portuguese.

With time expressions, passado normally comes after the noun:

• o ano passado
• a semana passada
• o mês passado
• o fim de semana passado

Putting passado before (o passado fim de semana) sounds wrong to native speakers.

They play different grammatical roles:

1. O fim de semana passado foi ótimo.

   • O fim de semana passado is the subject (“The last weekend”).
   • foi ótimo is the predicate (“was great”).

2. No fim de semana passado, fui à praia.

   • No fim de semana passado is a time expression (“Last weekend”).
   • Literally em + o fim de semana passado → no fim de semana passado.
   • The subject is implied “I” (fui = “I went”).

So:

• Use O fim de semana passado… when the weekend itself is the thing you’re describing.
• Use No fim de semana passado… when you want to say what you did during that weekend.

Yes, you can say O fim de semana passado foi muito bom, and it’s very common.

In terms of strength:

• bom – good
• muito bom – very good
• ótimo – great / excellent (a bit stronger than muito bom)

All are positive; ótimo usually feels a bit more enthusiastic or emphatic than muito bom.