certain
1.a certain man, certain persons, a certain Mr Carr等表达方式里的certain作“某”解，是形容词。certain of his friends里的certain作“某某若干”解，是名词或代词，属于复数，不可说a certain of his friends，该说a certain friend of his。
2.a certain hesitation, a certain coldness等表达方式里的a certain作“一些”解。注意hesitation和coldness都是抽象名词。
3.certain和sure有若干相似的地方，但certain比sure语气重些。两个单词后面都可以接of、不定式或以that引导的从句，如I am certain (或sure) of his honesty, I am certain (或sure) to see him tomorrow和I am certain (或sure) that he is honest。但It is certain that ...比It is sure that ...普通得多。
4.He is certain (或sure) to come tomorrow是“他明天一定来”。He is certain (或sure) of coming tomorrow是“他相信他明天一定来”。可以说It is certain (或sure) to be fine tomorrow，不可说It is certain of being fine tomorrow。
5.I am not certain (about) where he lives里的about可有可无。
