If it's any help, I can explain the difference between ~な and ～い adjectives the way my teacher explained it to me!
～い Adjective: You guessed it. These end in ～い. If you want to change an ～い adjective to a negative, you remove the い and replace it with くない. For example -- 高い (Takai/Tall or Expensive) becomes 高くない (Takakunai/Not tall or Inexpensive). People have already gone into how to say it was not or it used to be, so I won't re-hash. MOVING ON!
~な Adjective: These ones are tricky. There is a simple way to deal with them. Pay attention -- The ~な adjectives usually end in an え sound... け、て、め etc. Ke, Te, Me etc. BUT, BUT, BUT! There are some tricky ~な adjectives that have an ～い at the end. So whenever you see an adjective, ALWAYS look at the letter BEFORE the ～い to see if it's an え sound. If you find an adjective (for example KiReI), look before the ～い and if you see the え sound is れ and it is a ~な adjective.
Hope this helps! D:
I'm generally bad at explaining.