up

1.up（介词）不可用作“向着…上去”解，如不可说A cat jumped up the roof，该在up后面加上to或on，比较：

• A cat jumped up the ladder. （一只猫在梯子上跳上去［一跳一跳地上去］。）
• A cat jumped up to (或on) the ladder. （一只猫向着梯子跳上去［一刹那的动作］。）

2.up to（能胜任…）里的to是介词，后面可以接动名词，如I'm sorry, but I am so tired that I am really not up to going to the theatre with you tonight和He did not feel up to arguing with me that morning里的going和arguing不可改作go和argue。

3.Time is up是“时间完了”，不是“…的时间到了”，不可说Time is up for lunch或Time is up to go home。He looked at his watch to see whether his quarter of an hour was up是“他看着他的表，看他（所有）的一刻钟是否已经完了”。

4.关于up和down的区别，参见down 1