Complex sentence: Iago, Othello's trusted companion, envies Othello's life and plans to ruin it by manipulating Othello into believing that Desdemona is having an affair with Cassio.
Split into simpler sentences: Iago, Othello's trusted companion, envy's Othello's life and plans out a way to ruin Othello's life by manipulating Othello. Iago plans to make Othello believe that Desdemona is having an affair with Cassio.