按照语法规则：当主句与宾语从句之间插有其他成分时引导宾语从句的that通常不能省略。
如：
◆她答应要是谁找到他的儿子，她会给他一大笔钱。
正：She promised (that) she would give him a lot of money if anyone found her son.
正：She promised that if anyone found her son she would give him a lot of money
误：She promised if anyone found her son she would give him a lot of money

◆他说等他妻子回来，他就离开。
正：He said (that) he would leave when his wife came back.
正：He said that when his wife came back he would leave.
误：He said when his wife came back he would leave.
◆她在信上说她爱我。
正：She said (that) she loved me in the letter.
正：She said in the letter that she loved me.
误：She said in the letter she loved me.
欧美公学徐老师整理。