15.被动语态：1)被动语态的基本构成形式：主语+助动词be+构成形式2)被动语态的时态：被动语态的时态是通过助动词be的变化来体现。这个助动词必须与主语的人称和数相一致。3)主动语态和被动语态的结构相比(箭头表示动作方向)

主动语态：主语(动作发出者)→谓语(及物动词)→宾语(动作承受者)

被动语态：主语(动作承受者)←谓语(be+动词过去分词)←宾语(动作发出者)

He wrote the letter. →The letter was written by him.

4)主动句变被动句的基本句式：

a) 主：主语+助动词(情态动词)+谓语+宾语

被：主语(原宾语)+助动词(情态动词)+be+动词过去分词+by+原主语

如，We will help him.→He will be helped by us.

主：主语+动词短语+宾语

被：主语(圆宾语)+be+动词短语的过去分词+by+原主语

如，They looked after the babies.→The babies were looked after by them.

主：主语+谓语+间接宾语+直接宾语

被：主语(间接宾语)+be+动词过去分词+原直接宾语

主语(直接宾语))+be+动词过去分词+to(for)+原间接宾语+(by+原主语)

如，We gave him some magazines.→He was given some magazines.

Some magazines were given to him by us.

【注意】一般将主动句中表示人的间接宾语变为被动句中的主语。

主：主语+谓语+宾语+宾语补足语

被：主语(原宾语)+be+过去分词+原宾语补足语

如，They elected him their monitor.→He was elected their monitor.

【注意】如果主动句中宾语补足语是不带to的不定式，变成被动语态后一般要加to.

I saw him go to the church.→He was seen to go to the church.

【扩展】一般在以下几种情形时，需要使用被动语态。

不知道动作的执行者，只知道动作的承受者时。

When we got there, the room had been cleaned already.

不必要说出或出于礼貌不便说出动作的执行者时。

I was told that you were not honest enough.

3)有必要突出或强调动作的承受者时。

You were asked to answer this question.

16.if与whether:if和whether均可用于宾语从句中，意为“是否”;if用于条件句中，表示“如果”，不能和or连用;whether常与or连用，表示两种条件或情况。

【扩展】if not..如果不是…,表示惊讶或憎恶等情绪;if only意为“若…那就好了，我多么希望…”相当于how I wish;only..if..意为“决不…除非…”

If it hadn’t been for the doctor’s care, I should not be speaking to you now.如果不是医生的照顾，我现在就不会和你说话了。

If only he arrives in time!若他及时到达就好了。

I’ll only come if you promise me that you won’t invite Henry.除非你答应不请亨利，否则我不会来。

这就是我们为大家准备的初二年级下册英语第六单元知识点的内容，希望符合大家的实际需要。

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