他威胁要起诉该报社诽谤。

He threatened to ___sue the newspaper for libel.

如果我收不到退款，我将不得不起诉这家公司。

If I don't receive a refund, I will have to ___sue the company.

该公司决定起诉其前高管违反合同。

The corporation decided to ___sue its former executive for breach of contract.