Why can I not use 'function' for funktioniert? It seems to be the root word, and would play the same role - whether something 'functions' or 'works'?
Because 'my key is not functioning' doesn't make much sense in english, and sounds awkward.
Remember, cognates (words that sound alike) are dangerous guides.
Well, I disagree and think it does make sense, but agree that it does sound awkward. ie: "My key is not functioning" (where the expected function of a key is to open a lock, and this key is not opening the lock).
"To function" is one of the dictionary translations of funktioniert too: http://dict.leo.org/#/search=funktioniert&searchLoc=0&resultOrder=basic&multiwordShowSingle=on
Why is "My keys are not working?" not acceptable here?
If i am not mistaken: der Schlüssel (singular), die Schlüssel (plural) How do i know that the german sentence is singular or plural?
"mein" is used for masculine nouns, "meine" for feminine.