That is a literal translation, but in English, we would say, "to close and to open aren't hard", not that they "is not" hard. To close, on its own, is not hard. To open, on its own, is not hard. To close and to open (two infinitives), together, are plural, so, together, they aren't hard, rather than they "isn't" hard. Neither is hard, so both aren't hard.
I think it's extremely rare to start an English sentence with an infinitive (except for Hamlet of course, "To be or not to be..."). I understand that this lesson is about learning Romanian, yet, I think one shouldn't forget that sometimes you can't mirror a sentence, so it'd be more English-like to translate this e.g.: "It's hard to close and open"