您好,不好意思,
The problems are such difficult ones that we can't solve them by ourselves.
我错认为您说的是：加不加 difficult ones,一不一样.实在不好意思,误导您了.
加上difficult ones是为了方便理解,而不是加上them.
The problems are such that we can't solve them by ourselves.
是必须加them的,因为that是连词,后面的句子谓语动词的宾语是不能少的.
希望能帮到您.