Absolutely, in this sentence, either 'hard' or 'difficult' can be used to say 'not easy'. The trouble would arise when using 'hard' to describe the firmness or toughness of an object. For example, "Diamonds are very hard" uses this different meaning of 'hard'. Here, we would not be able to say, "Diamonds are very difficult".

Hey Izabela! Joe gave you some great advice there! In most cases if the word hard would NOT be used to describe an objects firmness (the Diamond is hard), then the word “difficult” can be applicable all the time and I would encourage you to say “the test was difficult” instead of “the test was hard.” Hope this helps :)

Agree with JOE. Here difficult works. Hard has 2 meanings.

Here they can be used interchangeably. It's just that "hard" has also the meaning of something firm, e.g. a hard chair. In this case "difficult" can't be used.