When is "ch" pronounced "k"?

 

So unfortunately, it is not a simple rule. It relates to the history and origin of a word.

When a word is of Old English or Germanic origin, it's pronounced "tch", like: 

• child, church, chest, chop, chair, chore

When a word is of Greek origin, it's pronounced "k", like: 

• character, chemistry, archeology, Christmas

Usually these words correspond to medical or religious terms. 

When a word is of modern French origin, it's pronounced "sh", like: 

• chemise, chef, chic (not chick), cliché, chauvinistic, chagrin

These words often correspond to fashion and food terms (évidemment !) 

Bonus: This reminded me of the word "schedule" and the fact the UK pronounce "sh" and the US pronounces "sk"; if we follow the logic above, the US is right, as it is from Greek origin)