foi...que

In Portuguese, the oração clivada (cleft sentence) is a powerful tool for emphasis.

1. Structure and Agreement:
The basic formula is: [Ser] + [Focused Element] + que + [Rest of Sentence].
In standard grammar, when the focused element is the subject, the verb ser must agree with it in person and number. However, when focusing a direct object, modern descriptive grammars state that the invariable 3rd person singular is standard (e.g., Foi os livros que eu comprei), and the verb ser NEVER agrees in person with a direct object (e.g., Foi a nós que ele convidou, not Fomos a nós). The verb ser also typically matches the tense of the main verb. If the focused element is preceded by a preposition or is an adverb (e.g., Foi ontem que eles chegaram), ser remains invariable in the 3rd person singular. In everyday spoken Brazilian Portuguese, it is extremely common for ser to remain invariable in the 3rd person singular even with subject pronouns (e.g., Foi eu que fiz, Foi eles que ligaram). Warning: In formal writing and proficiency exams, this is considered a strict grammatical error for subjects; you must make the verb agree (e.g., Fui eu que fiz).
Examples:

• Fui eu que fiz. (It was I who did it - subject focus).
• Foi os livros que eu comprei. (It was the books that I bought - object focus).
• Será ele que fará o discurso. (It will be him who will give the speech).
• Foi com eles que eu falei. (It was with them that I spoke).

2. The Invariable "é que":
While ser often matches the tense of the main verb, it is very common to use é que as a fixed, unchanging emphatic particle (locução expletiva) regardless of the tense or subject. The syntax here is: [Focused Element] + é que + [Rest of Sentence].
Example: Eles é que fizeram o trabalho. (They were the ones who did the work). Because é que acts as an expletive particle, it can be entirely removed from the sentence without changing the core meaning or making it ungrammatical (e.g., Eles fizeram o trabalho).

3. Interrogative Use of "é que":
A crucial and highly natural use of this structure, especially in spoken and European Portuguese, is in questions. Adding é que after a question word adds emphasis and flow.
Example: Onde é que você mora? (Where [is it that] you live?)

4. What can be focused?

• Subjects: Foi o João que ligou.
• Objects: Foi um carro que ele comprou.
• Adverbials: Foi em Paris que nos conhecemos.

5. Using "Quem":
When focusing on a person, you can replace que with quem. If the focused person is the subject of the embedded verb, the verb can either stay in the 3rd person singular or agree with the subject (both Fui eu quem fez and Fui eu quem fiz are correct). However, if the focused person is the object, the verb must agree with the actual subject of the clause (e.g., Foi a Maria quem nós vimos).

6. Prepositions:
If the embedded verb requires a preposition (like com or de), the preposition is placed before the focused element, leaving the relative pronoun que bare. For example, Gosto de você becomes Foi de você que eu gostei (not Foi você de que eu gostei). This rule also applies to the invariable é que structure: Com eles é que eu falei.

7. Pronoun Placement:
The word que acts as a trigger for proclisis, moving object pronouns before the verb: Foi ele que me deu o presente.

8. Pseudocleft Sentences (Orações pseudoclivadas):
A highly natural alternative to the standard cleft structure is the inverted pseudocleft sentence, which uses quem (for people) or o que (for things).
Examples:

• Quem ligou foi o João. (The one who called was João).
• O que ele comprou foi um carro. (What he bought was a car).