"I don't understand how you let him do that."
Translation:Je ne comprends pas que tu l'aies laissé faire ça.
I noticed this too: one is using the subordinating conjunction ("how") followed by the present tense "...pas comment que tu l'as..." and then in the alternative translation there is no subordinating conjunction followed by the subjunctive "...pas que tu l'aies..."
I also would like to know why "comment" doesn't seem to work. ...Or does it?
There is a nuance difference that we native English speakers get tripped up in. "Comment" is a question, in the context of "How do you call yourself?" or "How are you doing?" In the context of this particular sentence ("I don't understand how you let him"), the question is not "Which way did you let him do it," but rather "Why in the world would you let him do it."
When my students are stuck, I suggest to them that they think about how Shakespeare might say it. "I don't understand that you let him do it," sounds really archaic in English, but it gets you closer to the French. Does this help, I hope?
I'm neither a native french nor english speaker, but I have gone to school in french my whole life (almost, english at the end), and live in quebec (french canada) and "je ne comprends pas comment tu le laisses faire ça" is correct, at least here. "Je ne comprends pas que tu l'aies laissé faire ça." is "I don't understand that/how you had let him do that."
The original sentence was written in French with a past subjunctive, which means that the "let him do that" is past and complete. However, you are right, "let" is not specific, so I added 2 sets of other acceptable translations in the present tense:
- Je ne comprends pas que [tu le laisses/vous le laissiez] faire [cela/ça].
- Je ne comprends pas comment [tu le laisses/vous le laissez] faire [cela/ça].