I think … 后面一定要有that吗I tell sb. … 后面一定要有that吗

I think … 后面一定要有that吗I tell sb. … 后面一定要有that吗
I think … 后面一定要有that吗
I tell sb. … 后面一定要有that吗

I think … 后面一定要有that吗I tell sb. … 后面一定要有that吗
不一定,书面表达中一般会考虑到接一个that让读者读起来比较有层次感,但口语当中基本是省略的.比如：I think you're right.这句是平时很常见的,或者：I think I've been here before.这都是think 之后接比较简单从句的情况,这些情况下that一般会被省略.只有后面从句较复杂时,你或许会需要一个that来让读者感到意思更清晰.I think (that) being a mother is not so easy while you also have a full time job.此处that实际亦可省略,但为了表达清晰,也可以加入.
另外,在即时的口语表达中,有时说话人在说完I think 之后,脑子会在组织后面的语句,需要点时间停顿,这时会把一个that拖一点时间,这个that没有什么实际意义.