In sentences like "He taught them so that they would understand," if the speaker is certain that "they understand," could we use the normal past tense as Les enseñaron para que entendieron instead of entendieran?
No, you cannot.
You base things off of whether something has already happened at the particular moment in the narrative sequence. He taught them at moment X so that they would learn at some point after moment X. Because the very nature of the preposition para obligates such an ordering, it's impossible to use indicative after para que. As well, para que also hits another case for subjunctive: volition/desire. He does it in order that they understand (as you learn, you'll notice that many times, often times several of the reasons for subjunctive might apply simultaneously).
Now, your thinking isn't entirely off. If the preposition had been, for instance, después de, then you'd see a difference:
- Después de que les enseñó, entendían.
No subjunctive. The understanding comes after the teaching, so we know the teaching has occured.
- Después de que les enseñe, entenderán. Subjunctive because in this case, he has not yet taught them, and thus they still do not (yet) understand.
"para que", just like other purpose linkers like "a fin de que / con el objeto/propósito de que", needs to be followed by subjunctive because, at the time when the main action is/was performed, the result is/was not known:
Les enseño para que entiendan. (I teach you that you will understand.)
Les enseñé para que entendieran. (I taught you so that you would understand.)