手紙を鉛筆で書いてはいけません。

letter

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

pencil

indicates location of action; at; in; indicates time of action; indicates means of action; cause of effect; by; and then; so; indicates continuing action; indicates certainty, emphasis, etc.

to write; to compose; to pen; to draw; to paint

if ... then; when; and; why don't you...?; the more (one does something); because; since