Correct use the adjective “key”
I would like to know if it is correct to use the adjective “key” predicatively. I was taught that this word is like the adjective “main,” which can only be used in the attributive position. I’ve seen sentences like “This is key to the success of the plan,” but I remember typing something similar and the word processor marked it immediately as wrong. I think both “key” and “main” are special, (irregular, if you want) adjectives (in fact, they have no comparative forms) and feel they should be treated accordingly. I’ve never seen something like “This book is main in our course.” We will normally say “This is the main book in our course.” Thank you for your help!