It is sure that he has done it. (Make it simple)

• Complex sentence এ it is certain, it is surely, it is sudden থাকলে Simple sentence এ
- it is surely এর পরিবর্তে surely হবে + এর পরের relative pronoun টি বাদ যাবে + বাকি sentence টি বসবে।

• সুতরাং, নিয়মানুযায়ী, 
Complex: It is sure that he has done it.
Simple: Surely, he has done it.

• অন্য দিকে, 
- it is certain এর পরিবর্তে certainly হবে
- it is sudden এর পরিবর্তে suddenly হবে.

যেমন:
Complex: It is certain that he will come here.
Simple: Certainly, he will come here.

Complex: It was sudden that he lost his mobile.
Simple: Suddenly, he lost his mobile.

Source: Advance Learner's English Grammar and Composition by Chowdhury & Hossain.