I have been constantly told that だ can not be placed after an I-adjective. Why is that?
だ is the plain-form copula (the "is; to be" word). In the plain form, い adjectives already form a complete predicate (the piece of a sentence or clause that can complete that sentence or clause). In translation, it's like the い adjective already includes the "is" meaning -- so 速い would be "[it] is fast", not just "fast".
Since だ is only used to provide a way of finalizing plain-form clauses, it's redundant in cases where the plain-form clause is already a complete predicate -- such as when it ends in an い adjective. Saying 速いだ might be like saying "[it] is is fast" in English -- the meaning is clear, but it's grammatically wrong and it sounds wrong.
Grammatically speaking, there really are no adjectives in Japanese. i-adjectives are just special verbs. i-adjectives have many of the same inflections as do verbs, and they fulfill a grammatical role essentially equivalent to that of verbs. Therefore, 電車は速い is a complete sentence meaning "the train is fast", where 速い is the predicate.
i-adjective + です could be considered a special construction for the sake of politeness, similar to verb + ます. However, attaching an adjective to だ or でした, etc., would be ungrammatical or at least unidiomatic (because you would have two predicates). If you need the polite form of the past tense, instead of 早いでした, you would say 早かったです.
I know this is really confusing for many people. I used to be confused about the same question as well, but when I realized that i-adjectives are really a special category of verbs, it is easy to understand what is going on.
well, that is kind of truth for くん読みwords; there is really a verb 早う, rarely used today. And in the past, much more いadjectives had う,る, or ふ form. In contrast, な adjectives are 外来語 and follow Chinese contextual rules.