Question

what I could do was lie very still in order not to be discov

Answer 1

加和不加都可以,但后面都是不定式.

这是what + sb + can(could) + do 的特殊的地方.

例如：

All what I could do is be an ordinary friend to her.

What I could do is go to the Window menu and choose.

What I could do was only ask leave for him.

What I could do was to learn to respect her decisions and set aside my. distressed feelings.

Now, All what I could do was to hope that my flight take off in the safe.

两种方式都对.