Because gathered is an adjective in this example

He saw a crowd of people 'gathered' == He saw a crowd of people 'who had gathered'. In the 1st sentence on left hand side, "gathered" is an adjective because it is in the past-participle form, qualifying the noun 'people' (who had gathered). It is NOT a verb. In the 2nd sentence on the right hand side, "who had gathered" is an adjective clause, that serves the same purpose by qualifying the noun 'people'. -------------------------------------------------------------------------------- "He saw him 'wrote' ".... is wrong because 'wrote' (the past tense of 'write') is a VERB and hence can't qualify the pronoun 'him' as an adjective. The correct expression should be : "He saw him 'writing'"..... here 'writing' is in the present-participle form, working as a qualifier... Again if I say, "He saw him 'was writing'.... it is WRONG. Because here 'writing' is a VERB with a definite (past continuous/progressive) tense....so, it can't be a qualifier....hence, NOT an adjective. -------------------------------------------------------------------------------------------------- Hope you get the difference. For further clarification, pls study "Participle and Gerund" chapter in your English Grammar book.