I said him because it is more likely that it is he who has to explain why there is a strange size bra in his dresser.
Because Duolingo is being politically correct...? It could just as well be "his."
"I cannot" (No puedo recordar...) means that I do not have the ability to do it. "I do not" (No recuerdo...) means that it is something that does not happen and does not necessarily have to do with my capabilities. Maybe I don't remember the explanation because I have a lot of other things on my mind, but I could if I tried.
"Do not" is a straightforward negation of the verb.
"Cannot" is negation of the ABILITY to do the verb.
But you are right to pick up this point, native English speakers CAN be quite sloppy at making the distinction in sentences like "I don't/can't remember". We often give a second meaning to "can" as a shorthand way to imply doubt. In fact, I have just done so in the first sentence of this paragraph ;-)
This may be a regional habit, I'm a native English speaker from the UK.
Because "answer" = «respuesta»/«contestación». Someone can explain something without it being the answer to a question. For example, my first sentence might suffice as an answer, but these next two sentences are an explanation.
Probably because of the "can't." It is best to spell out all contractions because Duolingo does not always have all of the possible contractions programmed into its database.
My "I am unable to recall his explanation" had "unable" marked incorrect. Why so?
It appears SU translates to her by default on Duolingo. I haven't seen it being translated as anything else.
Really? Lol, well, it can also be translated as "his" or "your" (formal). Only context can distinguish.
The word is from to explain. Explanation is the translation. Sometimes the system doesn't recognize certain responses. For example, It likes can and cannot, not able and unable.