вставьте There is или It is ... difficult to learn the words, ... a difficult word in this text, ... something nice in this box, ... nice to see you again, ... a lot of snow in the forest, ... snowing now.

It is - в безличных предложениях. There is - конструкция местонахождения.

It is difficult to learn the words. It is a difficult word in this text. There is something nice in this box. It is nice to see you again. There is a lot of snow in the forest. It is snowing now.