It was my grandfather who bought the instrument many years ago可以写成my grandfather has bought the instrument many years 还有为什么一定要it做形式主语?直接my grandfather who bought the instrument many years ago不行吗?

It was my grandfather who bought the instrument many years ago可以写成my grandfather has bought the instrument many years 还有为什么一定要it做形式主语?直接my grandfather who bought the instrument many years ago不行吗?
It was my grandfather who bought the instrument many years ago可以写成
my grandfather has bought the instrument many years 还有为什么一定要it做形式主语?直接my grandfather who bought the instrument many years ago不行吗?

It was my grandfather who bought the instrument many years ago可以写成my grandfather has bought the instrument many years 还有为什么一定要it做形式主语?直接my grandfather who bought the instrument many years ago不行吗?
这个是强调句,强调的“多年前买了这台设备的人是我的祖父”如果去掉前面这个结构就不存在强调的意思了.
改掉之后就是“我祖父多年前买了这台设备”
体会一下汉语意思的不同,你就明白了.
强调句常用句型
　1. 陈述句的强调句型
　　It is/ was + 被强调部分（通常是主语、宾语或状语）+ that/ who（当强调主语且主语指人）+ 其他部分.
　　例子 It was yesterday that he met Li Ping.
　　2. 一般疑问句的强调句型
　　同上,只是把is/ was提到it前面.
　　例子 Was it yesterday that he met Li Ping?
　　3. 特殊疑问句的强调句型
　　被强调部分（通常是疑问代词或疑问副词）+ is/ was + it + that/ who + 其他部分?
　　例子 When and where was it that you were born?
　　4.not … until … 句型的强调句
　　A. 句型为：It is/ was not until + 被强调部分 + that + 其他部分
　　普通句：He didn't go to bed until/ till his wife came back.
　　强调句：It was not until his wife came back that he went to bed.
　　B. 注意：
　　此句型只用until,不用till.但如果不是强调句型,till, until可通用；因为句型中It is/ was not ... 已经是否定句了,that后面的从句要用肯定句,切勿再用否定句了.
　　5.谓语动词的强调
　　A. It is/ was ... that ... 结构不能强调谓语,如果需要强调谓语时,用助动词do/ does或did
　　Do sit down. 务必请坐.
　　Do be careful when you cross the street. 过马路时,务必（千万）要小心啊!
　　B. 注意：
　　此种强调只用do/ does和did,没有别的形式；过去时用did,后面的谓语动词用原形.

原句是个强调句型，强调买东西的人是“我祖父”。因而要用IT做形式主语。
改写句式是正确的。但是并无强调哪个部分的意思。

It was 'someone' who did something, or
Someone did something (simple Past tense).
My grandfather bought the instrument many years ago.
my grandfather who bought the instrument ..不行.

强调句的固定用法，记住就行了。1可以2不可以