用所给单词的适当形式填空。

1. The children were ________ because they didn't get Christmas presents. (happy)

2. How can you make yourself ________ if you cannot express yourself clearly? (understand)

3. December is the _______ month of the year. (twelve) 

4. The doctor's ________ made all of them sad. (dead)

5. I think this dictionary is ________ for your study. (help)