This is the 2860th most frequent Polish word.
kupiła
"She bought."
Here, 'kupiła' is used as a past tense verb in the third person singular form to indicate an action of purchase that Maria completed.
Maria kupiła książkę.
Maria bought a book.
In this sentence, 'kupiła' is a part of a dependent clause, referring to a purchase done in the past that is being explained.
Wyjaśniła, że kupiła to wczoraj.
She explained that she bought it yesterday.
'Kupiła' here is used in a question form to inquire about whether the act of buying has been completed by the female subject.