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This video gives a good explanation of the -er vs the -ier ending for comparative adjectives.   Essentially, the -er ending modifies an adjective and makes it more so.   If you are a Spanish speaker the -er ending is the same or equivalent of the modifier mas. 

So for instance, hot in Spanish is caliente.   To make it even more so, you say mas caliente.   Translated literally, that would be "more hot".   But a better way to do this in American English is to add the -er ending and simply say hott-er

Example usage:  "Today is hott-er than yesterday."

However!  When the adjective ends in a -y then to make it more so we DROP the -y ending and add -ier. 

Example:   This piece of jewelry is heav-ier than that one, so it costs more.

Okay?  Check out the video for some good detail and exercises.

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