I know that he is telling the truth. The underlined part is

I know that he is telling the truth. The underlined part is a/an - Noun clause.
- এখানে "that he is telling the truth" অংশটি 'know' verb -এর object হিসেবে কাজ করছে।

• Noun clause:
- যে সব subordinate- clause noun এর কাজ করে থাকে অর্থাৎ, subject, object, compliment, বা case in apposition- এর কাজ করে থাকে তাদেরকে বলে noun clause.
- Noun clauses are used when a single word isn't enough.

• একটি বাক্যের যেসব স্থানে Noun clause বসতে পারে -

1. Verb এর subject হিসেবে।
Example: That he has much money is known to all.

2. Verb এর object হিসেবে।
Example: I know that he has done it.

3. Verb এর complement হিসেবে।
Example: This is what I said.

4. Preposition এর object হিসেবে;
Example: I cannot understand the meaning of what he said.

5. Noun/ pronoun - এর apposition হিসেবে।
Example: The fact that he is a thief is clear to all.

Source:
- A Passage to the English Language, S.M. Zakir Hussain.
- Advanced Learner's Grammar and Composition by Chowdhury and Hossain.