Underlining is the inflected form of underline which means to draw a line underneath something especially to add emphasis or to underscore.
Underlying means when something or someone is lying underneath another in its basic and fundamental state.Example:
"There are underlying effects to the consequence of your actions".
Underlining is what you do when you draw a line under a word or phrase.
“Zac is underlining the key words in his notes tonight before the exam tomorrow.”
Underlying is an adjective that means the thing that causes something else. You often see it with ‘cause’ or ‘problem’.
“An underlying cause for many traffic fatalities is that the driver was impaired.”