Some Additional Examples of "Mujhey (Poora) Yakeen Hai" - Mujhey (Poora) Vishwaas Hai - I am sure - Lit Meaning - I do believe -
Yakeen is more on the Urdu Side
a Vishwaas on Hindi side But Both are used in Colloquial Langauge
Mujhey Poora Yakeen hai Ki... I am absolutely sure that ....
Mujhey Vishwaas Tha ki ... I was sure that....
Person A - Kya tumhe poora yakeen hai ki jo tum kar rahe ho...wo sahi hai?
- Are you sure that what you are doing is right?
Person B - Haan! Mujhey Poora Yakeen Hai - Yes! I am sure.
A little contribution to this already correct answer :)