This issue derives from the philosophy I employed very early on in the course. When the expanded tree comes, you'll learn about another tense called the aorist. The aorist tense is used when making general statements. Something like "A woman is a mother" is a general statement (even though it's not true: not all women are mothers), and so would require the aorist tense, not the present tense.
In the early days of creating this course, I asked that only fully correct translations be accepted for the present—and no translations that would actually be more appropriate for the aorist. This is why "The woman is a mother" (something that could be said in the moment about some specific woman) was accepted, but "A woman is a mother" was not accepted. I have since changed my mind, and am slowly going through old examples and accepting aberrant translations like "A woman is a mother" that are technically grammatically correct.
Sorry for the inconvenience!