Should the Highlighted "was" in this sentence be "were" instead?
So the rules assigning each kind of virus to a given safety level were laxer than some might think was prudent.
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
Presumably, you're wondering if it should be "was" or "were" because you have a plural noun, "rules". If the verb related to this noun, you would say "the rule was.." or "the rules were...". But that isn't the case in your example.
In your sentence, the verb relates to the degree of care with which safety levels were assigned (it is described as "lax"), and that 'degree of care' is singular, so you should use "was".
The business about some might think is unnecessarily distracting and syntactically irrelevant. Here's a chart showing relative frequency for the simplest phrasing I can think of in this area....
My guess is that native speakers who don't think about it are the ones primarily responsible for promoting the singular usage here, because I can't think of any logical or syntactic / semantic justification for it.
All I can say is that as a native speaker myself, I'd normally prefer the singular verb form - as do many others. And presumably that preference is in reality fairly strong, since I assume it's got little to support it apart from "idiomatically established" (which somehow holds its own against grammarians and pedants! :)