Adjectives that use "de", "�", or "en" in French?

It's awkward. The rules are a bit vague. For example it's easy to make the mistake translating "a cup of tea" into French as "une tasse de th�" which actually means a cup made of tea as opposed to the proper translation "une tasse � th�". Babelfish and Google Translate give the wrong answer (de) when you do an English to French translation and give au rather than � if you go the other way.