Not beginning a sentence with "but" is a rule of thumb. This is not a hard and fast rule. This is not a hard and fast rule. Rather, it's a simple guideline you should apply to avoid risking a common grammatical mistake, namely: using a sentence fragment . More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.