Fasting is a shield which protects us from Shaitan, Allah’s punishment and the Fire of Hell. However, we need to assure that the shield is not damaged in any way. Otherwise it won’t be effective in doing its job.
The actions that damage this shield and render it useless are sins like Backbiting, Lying, Evil Glance, Swearing, Nonsensical Conversation, Useless Arguments, Slander, Haram Sustenance, Dirty Jokes and other evil deeds.
Therefore, we must try our level best keep oneself far away from these sins in order to retain the quality of our fasts and ensure that they be our shield on the Day of Judgment.