Learn Portuguese (Portugal): A testemunha estava a dormir quando o homem bateu à porta. - The witness was sleeping when the man knocked on the door.

In European Portuguese, the past continuous is formed using the imperfect tense of estar (here, estava) followed by the preposition a and the infinitive of the main verb (dormir). This is exactly like the present continuous (está a dormir - is sleeping), just shifted to the past!

Why do we use two different past tenses (estava and bateu) in the same sentence?

This is a classic pattern for interrupted actions in the past. We use the past continuous (estava a dormir) for the background action that was already ongoing, and the simple past or preterite (bateu) for the sudden, completed action that interrupted it.

What if the witness was a man? Would we say o testemunho?

No, a testemunha (the witness) is a feminine noun that stays feminine regardless of the person's actual gender. The word o testemunho does exist, but it means "the testimony" (the statement given), not the person.

Why does à have a grave accent in à porta?

The accent shows that two words have merged into one. It is a contraction of the preposition a (at/on) and the feminine definite article a (the). When a verb like bater (to knock) pairs with the preposition a, and the next word is a feminine noun like porta, they combine into à.