I knowthat he will come. The underlined part is -

I know' হলো principal clause এবং 'that he will come' হলো subordinate clause. Typically a main/principal clause is made up of a subject (s) (a noun phrase) and a verb phrase (v). Sometimes the verb phrase is followed by other elements, e.g objects (o), complements (c), adjuncts (ad). [source: dictionary.cambridge.org]