1)
* If the target word is noun... (名詞の場合)
I am a child. ==> 私は 子供 です。
I was a child. ==> 私は 子供 でした。
* If the target word is adjective... (形容詞の場合)
I am happy ==> 私は 楽しい です。
I was happy ==> 私は 楽しかった です。
I think, these changes happen because of the target word's category are different.
2)
Also about "楽しい" and "です", please check below for your information.
* We can translate "I am happy", as both "私は 楽しい" and "私は 楽しい です".
* This is because, "です" has a another meaning which makes the sentence more soft and polite.
* The sentence "私は 楽しい です" is more polite than just to say "私は 楽しい".
* Therefore, usualy we say "私は 楽しい です" for "I am happy"
I believe that your teacher wanted to teach you "楽しいです"
not as a one word, but as a binding of two words "楽しい" and "です".
Hope this will be some help to you...
With best regard.
お礼
そうですか ありがとうございます。