The boy _______ we saw yesterday is john's brother .前面为什么不用t

The boy _______ we saw yesterday is john's brother .前面为什么不用that?若果从句中没有主语,也就
是直接动词,能用that引导吗?

定语从句中,关系代词作宾语时,可以省略.

第二种情况中,关系代词作主语时,不可省略.

注：that 可以指代人或物. 这里也可以用who引导
再问： 若果从句中没有主语，也就 是直接动词，能用that引导吗？
再答： 定语从句中，不可能有无主句啊。引导词本身就作主语。如： Some people who are successful language learners often fail in other fields . 有些在语言学习上很有成就的人,在其他领域常常一无所成。
再问： Some people who are successful language learners often fail in other fields . 就是这种句型，who后面直接加are，像这里能用that吗？
再答： 可以的。that 代替人或物皆可
再问： 都可以，除了特定的前面有anyone不定人称代词，介词+关系代词，用逗号隔开的，，，还有什么不用that的？？？
再答： 一、关系代词直接放在介词后面时，要用which。例： Read the passage to which I referred in my talk. 读一下我在演讲中提到过的那段文章。 His new car, for which he paid ￡7000, has already had to be repaired. 他花700英镑买来的那辆新车已经不得不修理了。 二.which还可以用作关系限定词，后跟名词，that则不能这样用。例： He changed his mind for the second time, after which I refused ever to go out with him again. 他第二次变卦，以后我再也不跟他出去了。 The postman comes at 6.30 in the morning, at which time I am usually fast asleep. 邮递员早晨六点半就到，而这时我通常还熟睡着呢。 He may come, in which case I'll ask him. 他也许会来，来了我就问他。 There are 2 left, one of which is almost finished and the other of which is not quite. 剩下两件事，一件快做完了，另一件还有得做。 三、引导非限制性定词从句时，一般要用which，不用that。例： Football, which is a popular game, is played all over the wofld. 全世界都踢足球，它是一项很受欢迎的运动。 She spent all evening talking about her latest book, which none of us had ever heard of. 她花了整个晚上谈她最近的新书，关于这本书，我们没有一个人听说过。