'Hard' in the Bible

And I gave my heart to searching out in wisdom all things which are done under heaven: it is a hard thing which God has put on the sons of men to do.

There is an evil which I have seen under the sun, and it is hard on men;

Who is like the wise man? and to whom is the sense of anything clear? A man's wisdom makes his face shining, and his hard face will be changed.

Then I saw all the work of God, and that man may not get knowledge of the work which is done under the sun; because, if a man gives hard work to the search he will not get knowledge, and even if the wise man seems to be coming to the end of his search, still he will be without knowledge.