What is the difference between likely and certain?

Likely is less definite. For example, if the sky is dark and cloudy, it's likely that there will be a thunderstorm, but it might just be rainy instead of the thunder and lightning.

Certain is absolutely positive. For example, you could say,"I'm certain I'm a human" or "I'm certain I have a cerebral cortex (brain)".

"Likely" thinks it will probably happen.

"Certain" knows it will definitely happen.

Hope that helps, from PNE.