歯を磨かなくてはいけない。

tooth

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.

to polish; to shine; to brush (e.g. teeth); to refine; to improve

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