when should this be used and not be used?can someone explain the purpose of this feature.also when should i use interlace and de interlace in the tempenc settings and what does each of these features do?

thanks everyone has been very helpful so far

The ForceFilm option of DVD2AVI should be used primarily with an NTSC source it will reduce the frame rate from 29.97 to 23 fps. It greatly reduces the final encoded KVCD size.
De-interlacing is a very complicated subject, and I don't know if I could explain it with out be long winded.
Doom9.net have a very thorough tutorial here on both DVD2AVI and de-interlacing via IVTC in Tmpeg

http://www.doom9.org/mpg/dvd2avi2.htm
http://www.doom9.org/mpg/tmpg-ivtc.htm

It explains about the force film option and how to go about IVTC selecting the correct field orders and frame patterns to de interlace properly. Hipe this helps.