"Quando i fiori muoion" only translates to "when the flowers die". "Quando muoion i fiori" would instead translate to "when do the flowers die" as well as "when the flowers die." You need to listen to the voice intonation with this to understand which a person means. slight difference but makes all the difference
Actually, yes, it does work grammatically, as Italian does not have a separate present progressive form the way English does.
You can also translate "Io scrivo" as "I write" or "I am writing", "tu lavori" as "you work" or "you are working", and "loro parlano" as "they talk" or "they are talking." If Duolingo didn't accept it, then it should be reported.
morire. see the conjugations here https://konjugator.reverso.net/konjugation-italienisch-verb-morire.html
quando morivano. check here https://konjugator.reverso.net/konjugation-italienisch-verb-morire.html. However, this is the lesson on the present, not the past.
Sooner or later, the biological clock begins to run down. Cells that had faithfully renewed themselves begin to fail. A heart that pounded away in perfect synchrony begins to run down after a couple of billion beats. Skin that bloomed in the spring sunshine begins to weather and flake in life's autumn. Brains shrink, spines curve, eyes begin to fail, hearing goes, organs become cancerous, bones begin to crumble and memory perishes.