I never thought of it , let alone did I do it. let alone后为什么接did

let

alone

是连词，

意思是，更别说...。

did

I

do

it是部分倒装的用法，你可以参考更常见的nor，

I

didn't

think

of

it,

nor

did

I

do

anything.

另外这里let

alone后面可以直接跟动词，省略主语I：

I

never

thought

of

it,

let

alone

did

it.