Ответ:

1) bigger

2) difficult, the most difficult

3)worse, the worst

4)slower, the slowest

5)happy, the happiest

6)expensive, more expensive

7)sweeter, the sweetest (не точно)

8)more important, the most important

9) strong, the strongest

big - bigger - the biggest

difficult - more difficult - the most difficult

bad - worse - the worst

good - better - the best

slow - slower - the slowest

happy - happier - the happiest

expensive - more expensive - the most expensive

sweet - sweeter - the sweetest

important - more important - the most important

strong - stronger - the strongest