It was my grandfather who bought the instrument many years a

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