IPA: /ˈsɜːrtənli/
KK: /ˈsɜrtnli/
In a way that shows complete confidence or certainty; without doubt.
She will certainly help you with your project.
Certainly → It is formed from "certain" (from Old French *certain*, meaning sure or confident) and "-ly" (meaning in a manner of). The word "certainly" means in a manner that is sure or confident.
Think of being "sure" or "confident" (from "certain") and how the suffix "-ly" indicates a manner. This helps you remember that "certainly" means in a sure manner.