be动词有哪几种形式

当连系动词be表现为现在时态或过去时态时，其否定形式通常为be+not（从未如此）。若句子中蕴含情态动词，则not应置于情态动词之后。

闪光的未必都是金子。All that glitters doesn't necessarily mean gold.

只要愿意改正，永远都不晚。It's never too late to start anew and correct your mistakes.

在旧金山收听到了演说的转播，但不确定是否为实时直播。The speech was broadcasted in San Francisco, but it remains unknown whether it was live or not.

如你所知，这不是一个新难题。As you're aware, this issue isn't new.

无法用金钱来衡量它。It cannot be measured in monetary terms.

沙子必须经过筛选才能添加到材料中。The sand cannot be added to the material without being properly screened.

除非进行面谈，否则无法讨论此事。The matter cannot be discussed without a personal interview.

当你的母亲竭尽全力让你开心时，你不能对她冷漠。You cannot be unkind to your mother when she has gone above and beyond to please you.