Actually, "tabetaku arimasen" and " tabetaku nai desu" just show your negative to eat, like saying you don´t want to eat. And it´s not much of a polite declining since it would sound rude. To politely decline an invitation you could say　’’ちょっとですが。。。。。すみません。’’(Chotto desu ga.....sumimasen) or ’’ちょっと用事があるんですけど’’(Chotto youji ga aru n desu kedo) to say that you can´t or you have other matters to attend to. For the past tense you say "tabetaku nakatta desu". Hope it helps! By the way, there aren´t ignorant questions, for ignorant is that who doesn´t want to learn ;)