根据句子意思，用括号内所给词的适当形式填空。
The book with a blue cover is one of the __________ (twin).
At the end of that cold winter day, they __________ (decide) to go for a walk after dinner to relax.
I finished my Math homework an hour ago, but I forgot __________ (write) my English homework.
There is nothing wrong with __________ (keep) a snake if you like.
These __________ (video) are made by some students as a gift for their teachers.