I get the difference between words like たのしい and たのしく well enough, I think. Now, I confused where this new form comes in. What's up with たのしかった? Can anyone explain?
EDIT: typo correction
たのしかった is the past-tense form of たのしい, meaning "was fun."
So たなしいでした would be wrong?
Yes. It sounds right to an English speaker, but is not correct.
To add on to Cici (who is absolutely correct) in Japanese, most adjectives are conjugated by tense, not their verbs. Tanoshii becomes tanoshikunai when negative, and tanoshikatta when past tense.
To complete your answer, only "i" adjectives are conjugated ;)
Just wonder if the difference between 楽しい and 面白い same as in English, between "funny" and "interesting"?
I think 楽しい means more like 'fun, enjoyable' and 面白い means more like 'interesting ,intriguing'