The problem was too difficult for me to work out.改为复合句

这是一个 too……to……的句型，表示太怎么而不能怎样，我们可以用so……that……的组合来将其改为一个结果状语从句，也就是复合句中的一种：
The problem was so difficult that I couldn't work it out.
希望对你有所帮助~~

The problem was so difficult that I couldn't work it out.

It was too difficult for me to work out the problem.
(上面的，it was 是不能简写的）