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.