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Questions & Answers about A castração foi ontem.
Why is it a castração and not just castração?
Because castração is being used as a specific event: the castration.
In Portuguese, it is very common to use the definite article with nouns in places where English might sometimes omit it. Here, a castração refers to a particular procedure already known from the context.
Also, castração is a feminine noun, so the article is a, not o.
How do I know castração is feminine?
What does foi mean here?