When adding the suffex ing to word glide drop the?

When adding the suffix "-ing" to the word "glide", you drop the "e".

This is because the word "glide" ends in a silent "e", and the rule for adding "-ing" is to drop the "e" if the word ends in a silent "e" and the suffix begins with a vowel.

So, "glide" + "-ing" = gliding.