I-adjectives modifying a noun

Hi guys.

Okay, so you have your positive form i-adjectives which modify nouns, right, like 'yasashii hon' or something. Well I was going over negative versions of these, and instead of 'yasashiku nai hon' I said 'yasashiku arimasen hon'. Now my teacher told me this was incorrect, and I'm fine with that, but I coundn't seem to get my mind around the explanation given.

I would really appreciate a nice, simple but comprehensive explanation for this if possible.

Thanks in advance.

It's for the same reason as this:

Can we say ~しますつもり~ ? | Japan Forum

You do not normally use polite forms in modifying clauses (things that go in front of nouns).

So
この本は易しいです。
kono hon wa yasashii desu = this book is easy
but
これは易しい本です。
kore wa yasashii hon desu = this is an easy book
and you wouldn't use "yasashii desu hon".

"arimasen" is a polite form of "nai" here, the same as "yasashii desu" is a polite form of "yasashii", so although "yasashiku arimasen" is not incorrect grammar by itself, you don't use it in front of a noun.


The same applies if you were using a longer modifier, like a sentence (which you probably haven't come to in your class yet, but you can use other things in front of nouns in a similar way to a simple adjective).

For example, you can say:
妹とパイを作りました。
imouto to pai wo tsukurimashita = I made a pie with my sister.

If you wanted to say "I ate the pie I made with my sister." then we have to make "tsukurimashita" into plain form:

妹と作ったパイを食べました。
imouto to tsukutta pai wo tabemashita.

Even though we use a plain form, "tsukutta" in the middle, the whole of the sentence is the same level of politeness as the others ("masu/desu") because we use "tabemashita" at the end. "imouto to tsukutta" acts pretty much like an adjective, just describing the sort of pie I ate.