综述：
alternate指轮流的（happens or appears by turns），alternative则指供选择的（having a choice），但现在美国也用alternate指选择（例：offer alternate selections, taking an alternate route when they drive, alternate technologies，但英国人在上述语境中依然使用alternative）。
*alternate
adj.
-one after the other, alternating in a regular pattern（交替的）:
at alternate periods
-every other, every second (one)（间隔的）:
on alternate weekends
n.
-someone who alternates with somebody else in doing something
-a substitute or reserve for someone else（替补）
named first alternate to sb.
*alternative(comes from the Latin for "the other of two")
adj.
-one of only two possibilities (traditional)-->choices between two or more things (modern)
-something that you prefer to something else, or that you at least accept as a choice:
alternative energy sources（替代能源）
alternative medicine（替代医学/替代疗法）
alternative rock （另类摇滚）
n.
-a possible choice between two or more things:
once they have exhausted all other alternatives
# Keep in mind that the word alternative may suggest an obligation to make a choice; also, saying that something in one alternative may imply that it's a good choice, or at least that it's good enough.
来源：May I Quote You on That
