What is the subject-verb agreement here?

Post date: 2021-06-18 17:00:12

I want to know the grammatical reasoning behind this: a) Each cat has a collar b) Each cat must have a collar Why does adding must change the subject-verb agreement? This is next to impossible to google an answer for as I'm not sure what grammatical rule/exception I'm looking for, and it is slowly driving me mad not knowing why.