chắc hẳn

Meaning

1. surely

   adverb

   surely, must (be)

   Surer than chắc and more careful; common in writing.

   • Chắc hẳn chị mệt lắm.

     You must be exhausted.

   • Chắc hẳn anh ấy biết rồi.

     He must know already.

   to recognize