Math is a very recondite (hard to understand) subject.
it will be hard to understand for these people
It was hard to understand his Scottish brogue.
Alliteration is hard.
lol its hard
Using context and previous knowledge.
i don't understand so sorry
Societies is hard word to put in a sentence.
It is impossible to understand the violence of radical fundamentalists.
The test was painstakinly hard.
It is hard being a parent.
A reporter works very hard.