Your title is written just fine. In the message, though, you should write: "I can't tell the two apart." When considering 'would' and 'could', think of 'will' and 'can'. 'Will' discusses the intent or desire to do something. 'Can' discusses only the ability to do something. "I will wash the dishes if I can." - if it's possible to wash the dishes I intend to wash them. "I would wash the dishes if I could." - same as above, except using 'could' suggest that the act may not be possible. There's more tricky stuff. The two words are often used to ask questions, such as: "Would you buy me an ice cream?" or "Could you buy me an ice cream?", although these are pretty much the same request. Hope that helps. Could someone else jump in and add some clearer information?