When should you use soft c?

C is pronounced /s/ when followed by an i, e, or y.  We call this a soft c. 

Our language is Latin based. In early Latin there was no soft c, but in late Latin the two sounds of c developed because of how the language was being spoken, much like slang words evolve today. 

 There are exception words of course like:

•  Cello (from Italian like many musical words) 
•  Celtic (the Irish language has no letter K so all C’s are pronounced hard 
•  Soccer (the word soc was a shortened slang word for Association football then the suffix er was added) 

and a few others...

The rule “soft c before e, i, and y “ has to be learned to be able to decode many words.