is是什么时态?

例句：

New York is where the action is.

纽约是个热闹活跃的地方。

Today is Tuesday, so tomorrow is Wednesday.

今天是星期二，那么明天就是星期三。

It is a mild morning and the sun is shining.

这是一个和煦的早晨，太阳在照耀着。

The clear inference is that the universe is expanding.

显然结论是宇宙在扩大。

It is essential that your diet is varied and balanced.

重要的是你的`饮食应当是多样而平衡的。