Verb Tense Sequence

"As long as the main clause's verb is in neither the past nor the past perfect tense, the verb of the subordinate clause can be in any tense that conveys meaning accurately. When the main clause verb is in the past or past perfect, however, the verb in the subordinate clause must be in the past or past perfect."

What do they mean, "convey meaning accurately".

And can someone explain to me why "past and past perfect" subordinate clause tenses have to be that if their main clause is as well.

Here's a sentence.

"I want to go to the mall before it closes/closed"

"I want to go the mall after it closes/closed"

Mhmm, I know the present tense sounds better in the sub. clause. But why? Please and thanks!