I just don't get it

I saw the following sentence in a highly acclaimed book:

"For this reason, is it critically important that every investor understand how to measure both the explicit and the less transparent elements of investment transactions and management costs."

Shouldn't "understand" be in third person singular "understands," since it's an action that is performed by the singular "every investor"?

I believe that is "understand" is in the subjunctive mood and that is why it has no "s" on the end.

Yes, this is an example of the subjunctive in English. For example "It is critically important that every investor review his portfolio" However except for English professors nobody would bat an eye if you didn't use the subjunctive. It's disappearing in English, unlike in French.

I know "It is important that every person understand the reason behind etc." is correct and idiomatic in American English, but is the version without the subjunctive common and idiomatic as well? (In American English?) Ex:
"It is important that every person understands the reason behind etc."
Thanks.

The answer Johnny is that the subjunctive is in better health in the US than in the UK. The subjunctive version is correct, but might not be often run into in England.

i can't speak english. who can help

"I know 'It is important that every person understand the reason behind etc.' is correct and idiomatic in American English, but is the version without the subjunctive common and idiomatic as well? (In American English?) Ex:
'It is important that every person understands the reason behind etc.'"

Generally, American English (AE) continues to use the verb form without the third person "-(e)s" ending to indicate the subjunctive mood.

Increasingly, British English (BE) does not.