Provided this is correct:
The general rule is to match the tense of a subordinate clause with the tense of the main clause. E.g. He vacuumed while I washed the dishes. However, a subordinate clause can use present tense to express a general truth despite the main clause being in past tense. E.g. He discovered that the sun rises in the east.
What I'm wondering is:
When used in a work of fiction set in past tense, should present tense be used in a subordinate clause expressing a general truth?
The exact sentence that's sent me down this path is (in the context of a middle school health class): She explained how young girls bled once a month. I changed it to "bleed," but the author changed it back to "bled." Perhaps either is acceptable?