Urgent Grammar Question

[HannahG]

Please advise why using the word focuses in the following sentence below is incorrect. Thank You!

"I'm studying sentences that focuses on plural and singular verbs."

[Basharuae]

Hello there,
There is no problem with the word itself, but there should be an agreement with the subject (sentences) which is (plural)
so
I'm studying sentences that focus on plural and singular verbs
or
I'm studying a sentence that focuses on plural and singular verbs

Regards

[HannahG]

Thank You Basharuae!