"The test yesterday was not easy at all."
The first is more Formel.
The second is less formal. Though I don't think you use desu in short Form
I am not entirely sure, but I think you can say kuarimasen instead of kunai, too. I think it was a polite version or something, but I am really not sure.
Because you don't use adjectives in front of verbs. That is why you make yasashii into yasahiku
Yes I forget the text. Well in the negative Form you have to change II adjective to adverb.