各种时态的时态信号

一般现在时：all the time, always, often, usually, never
一般过去时：yesterday, last year, 50 years ago
现在完成时：since, for (how long), ever, before
进行时：在时间的一点上， 如，at 5 o'clock, now,
将来时：in the future, next year, tomorrow, next week, in 10 years.