gerund or infinitive

http://a4esl.org/q/f/z/zz97mkm.htm

Modals

http://a4esl.org/q/h/0101/ig-modals.html

The ING form

http://www.better-english.com/grammar/theingform.htm

Must and Don't have to

http://www.better-english.com/grammar/mustnt.htm