It is certain that he will win the match.由于没有上下文，只就本句分析。首先it如果是实指，要搞清楚它代替的时什么。对于本句，显然理解成 它代替的是 that he will win the match这句话比较合适，因此，这里是主语从句，it是形式主语。已形成固定用法和译法的“it”作形式主语的主语从句结构有些用“it”作形式主语的主语从句结构，已形成固定用法和译法。常见的有下面4种：1）It is+名词词组+从句It is a fact that ... 事实是...It is good news that ... ...是个好消息It is a question that 完难望... ...是个问题It is common knowledge that ... ...是个常识2）It is +形容词+从句It is necessary that ... 有必要...It is clear tha限评派将粉迫府因国合西t ... 很清楚...It 量倍脸任去哪常刚却河is (un) likely that ... 很（不大）可能...It is important that 来自... 重要的是...3）It is+过去分词+从句It is said that ... 据说...It is reported that ... 据报道...It has been proved that ... 据证明It must be pointed out that ... 必须指出...4）It is+不及物动词+从句It seems that 液正示良机标连航占... 好像是...It 鲜料径变happened that ... 碰巧...It foll360问答ows that ... 由此可见...不要将强调词“it”和形式主语“it混淆起来。强调词“it”引导强调句，对句中某一个成分（大多为主语、宾语和状语）进行强调。有些动态形容词如：sure（确信），certain（肯定），afraid（恐怕），confident（确信）等，用作表语时，可跟一个意义上相当于宾语的名词从极法句。上面这段就是解释了你问题中的第二个句子： I am certain that he will win the match.上面的句子中that he will win the mathc为表语形容词certain的宾语。It is certain that he will win the match. 开黄英却曲I am certain that he will win the match.这两句话表达的重点还是不一样的，第一句侧重于描述一个事实，许多人据协都德认演及的看法。第二句只是主语的看法。
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