Make 'haec rēs atrōx' accusative singular.

The phrase "haec rēs atrōx" translates to "this terrible thing" in English, where "haec" is the nominative singular feminine form of the demonstrative pronoun, "rēs" is the nominative singular feminine noun for "thing," and "atrōx" is the nominative singular form of the adjective meaning "terrible."

To change this phrase to the accusative singular, you need to ensure that all parts match the accusative case. The accusative singular feminine form of "rēs" is "rem." Additionally, the demonstrative pronoun "haec" must agree with the noun in case, number, and gender, and thus changes to "hanc." The adjective remains in the accusative singular form as "atrōcem" to agree with "rem."

Putting all of this together, you arrive at "hanc rem atrōcem," meaning "this terrible thing" in the accusative case, which is the correct transformation of the original phrase.