- ButwhatIcantellyou,isthatithappe?
- But what I can tell you,is that happens in the blink of an eye.
is是不是多余的
- is是不可少的.
这个句子里有一个主语从句和一个表语从句.
what i can tell you是主语从句，充当主语．
that it happens in the blink of an eye是表语从句，充当表语．
一个完整的句子必须有主系表结构，有了主语和表语，还要一个系动词，用is.
不知是否明白？