Question

it was something that was completely inside my head.此句是定语从句吗?

Answer 1

1 可以理解为定语从句：
It was something that was completely inside my head.它是完全在我的脑海里的东西。
It是代词作主语，指代某事物。something是先行词，that是关系代词，在定语从句中作主语。
2 也可以理解为强调句：
It was something that was completely inside my head.是某东西完全在我的脑海里。
It无意义，引导强调句型。强调something: Something was completely inside my head. "某东西完全在我的脑海里。"
强调句型的结构：
It is/was + 强调部分 + that（强调部分指人时也可以用who) + ……
究竟是上面的哪一种意思，要根据此句的上下文来确定。

Answer 2

是定语从句。

something是先行词

that是引导词

that was completely inside my head是定语，也就是从句部分。

【定语从句举例】This is the book which interests me.（“which interests me”是关系从句，修饰先行词“book”，而关系词“which”指代先行词“book”并在关系子句中充当主语。这句话可以拆分为两个句子来理解：“This is the book.”和“The book interests me.”）