hard/tough nut to crack
sth．difficult to understand or to do 难事；棘手的问题
The girl found the algebra lesson was a hard nut to crack．这女孩发现代数课难以应付。
Dealing with that problem was a hard nut to crack；you must have worked hard to solve it．处理那个问题很难，你一定花了很大的气力。
hard put（to it）
in a difficult position；faced with difficulty面临困境；为难
The little girl found herself hard to put to it to find the way home．这小女孩发现她认不得回家的路了。
We were hard put to it to solve this problem，but we succeeded at last．我们起先对解决这个问题感到很为难，但最终还是圆满解决了。