1.you are completely right 因为complete是形容词,此处需要改成副词
2.he is a famous writer,but he is better at playing football.
be good at＋doing,所以把play加上ing
3.i'll hand in my paper if it is possible.
这个句子不完整,必须加上it才是主谓宾,否则没主语
4.people are not allowed to smoke in the factory.
be allowed to是固定用法,而不能是ing
5.the students were very tired after they finished too much homework
too much ＋不可数名词,固定用法,指的是太多的XX东西