I knowthat he has done it. The underlined part is -

♦ I know that he has done it. The underlined part is - Noun clause.

• A noun clause is a group of words that band together and act like a noun.
- Nouns clauses are used when a single word isn't enough.

• একটি বাক্যের চারটি স্থানে Noun clause  বসতে পারে-
1. subject হিসাবে 
2. transitive verb এর object হিসেবে
3. Linking verb এর পরে complement হিসাবে 
4. Preposition এর Object হিসেবে।

• I know that he has done it.
- এই বাক্যে 'that he has done it' verb এর object হিসেবে ব্যবহৃত হয়েছে। 
- এটি একটি Noun clause.

Source: A Passage to the English Language by S. M. Zakir Hussain.