When not connected with a noun, 'much' is commonly used ONLY in negative sentences and questions. For example: She did not speak much yesterday. Did she speak much yesterday? 'A lot' is commonly used in positive sentences, as well as in negatives and questions -- so basically in all types of sentences. She spoke a lot yesterday. She didn't speak a lot yesterday. Did she speak a lot yesterday? If you don't want to worry about when to use what -- always use 'a lot' and you will always be correct!
Duolingo preferences aside, i do not (completely) agree with you. "much" can very well be used in non-negative contexts. Examples: "There still is much to learn." (neutral) "He achieved much in his life." (positive) "Much happened since we last spoke." (neutral)
"His performance left a lot lacking." (negative!)
Yes, one could substitute "much" with "a lot" very often, but it might be more a question of style.
I wrote 'commonly', not 'exclusively' and I am referring to grammatically negative sentences (those containing 'no' or 'not') and not sentences with negative meanings or a negative context. Example: I killed Billy. (positive) I did not kill Billy. (negative). As I've taught English to hundreds of Germans, I understand that you all can easily confuse 'emotionally negative' or 'contextually negative' for grammatically negative, but they aren't the same thing. "It is raining today' is not a negative sentence simply because rain depresses Germans and 'that's bad'. :-)
To me, "His performance left a lot lacking', is a very awkward and non-idiomatic sentence. I'd think you meant: His performance left a lot to be desired. But that's possibly not what you intended. (Oh, and it's Much HAS happened since we last spoke. Present Perfect.)
I stand by my claim that to sound like a native speaker, learners of English can follow the rule given above and always be correct in any standard situation. That doesn't mean it's the only correct form, but that native speakers themselves prefer that use over all others. Much is often used in formal situations, not standard situations. So, I'd say it's more a question of register than style.
Look at this example: She spoke much, but said little. (very formal, rhetorical or poetic in tone. ) And now...She spoke a lot, but didn't say much. (modern, standard colloquial for both British and American English)
Both of these sentences are grammatically correct, but the first one will get you strange looks if you use it in standard situations because the use of much here is FAR too formal. Formulations with much in grammatically positive sentences (as in your examples) belong more in the realm of academic papers, official letters, and formal events like funerals or political speeches -- and, if you like, in the English of older, very proper British women, who place a lot of emphasis on respectability -- and not in the register of common language.
Thanks for the clarification. I was not aware that you were referring to grammatically negative sentences. And yes, my example "His performance left a lot lacking." is awkward. That happens when you think more about trying to cobble together an example rather than actually trying to express a genuine thought. ;)