、。
She said yesterday that she was sick, but it wasn't true.
Literal
She [topic-は] yesterday sick [is-だ] [quotative-と] said [but-が], that [topic-は] true [genitive-の] thing was-not.
Direct quotation in Japanese keeps the embedded clause's casual だ intact (病気だ rather than the polite 病気です), with 言った framed as a verb of speech — a structural mismatch with English, where reported speech would normally drop or convert the copula. The clause-final が works as a soft 'but,' setting up the correction in the second clause. ではなかった is the plain negative past of the copula (だ → ではない → ではなかった); contracts to じゃなかった in casual speech and rises to ではありませんでした in polite. The whole sentence is a tidy three-stage frame: claim → contradiction marker → reveal.