to use; to make use of; to employ; (coverb) with; using; to need; to have to (usu. used in the negative or in questions); usefulness; utility; (bound form) expense; outlay; (courteous) to eat; to drink; (literary) hence; therefore

need not

useful

to use; to employ; to apply; to make use of

to act on; to affect; action; function; activity; impact; result; effect; purpose; intent; (suffix) -ation, -tion etc., as in 抑制作用, inhibition

to exploit; to make use of; to use; to take advantage of; to utilize

useless

cost; expenditure; expense