In Japanese, you always need a main verb to complete a sentence. If you directly attach an i-adjective to an auxiliary ください
via て
, then you will not have a main verb anywhere. You need at least one main verb in between:
温かくしてください
優しくしてください
In sentences with i-adjectives or copula, it may not be obvious that you are using a verb, but underlyingly, you do have a verb. For example, the ordinary ending of an i-adjective is い
:
寒い
This い
includes a hidden verb ある
, which is not obvious because the sequence くある
is contracted to い
. If you place something that interrupts the whole hidden sequence, thereby preventing the contraction, then you can see that there is indeed a hidden ある
:
部屋は暗いだけでなく、寒くもある。
× 部屋は暗いだけでなく、寒くもい。
× 部屋は暗いだけでなく、寒いも。
With nouns or na-adjectives, you may end a sentence with だ
, but that is a contracted form of である
, which includes the verb ある
. Again, it may not be obvious that だ
is the contracted form of である
, but if you place something that interrupts the whole hidden sequence and prevents the contraction, you can see that there is indeed a hidden ある
:
彼は生徒でもある
× 彼は生徒もだ
× 彼は生徒だも