is there a rule of thumb to distnguish when a word ends with "ence" and when it sends with "ance". For example, existence and importance. Sometimes I encountered words and I was puzzled about whethere to put e or a and I had to check teh dictionary to know.
The short answer you've already discovered: The Dictionary.
The long answer is probably more than you want to know. It has to do with the preceding letters and sounds. When it's a hard c or g for example, it's -ance (significance, arrogance, etc); when soft, it's -ence (innocence, intelligence). Still interested? It gets worse: "Y" roots add -ance (defy > defiance), -ist words add -ence (existence) and it goes on and on.
Yep. The Dictionary.