1.常用that 不用which的情况:1）先行词为不定代词all,little,few,much, anything, everything, nothing, something等时.I will do all (that) I can to help you.2）先行词被序数词、形容词最高级或the only, the very等所修饰或其本身就是序数词﹑形容词最高级时.He was the only person that joined the army in his village that year.3）先行词既有人,又有物时They talked about the people and the things (that)they remembered in the school.4）当主句是以who 或which开头的疑问句时, 定语从句用that 引导, 以避免重复.Who is the boy that was here just now?