You cannot Pendulum Summon, except LIGHT monsters (this effect cannot be negated). If you do not control any face-up monsters, other than "Melodious" monsters: You can add 1 "Melodious" Spell/Trap from your Deck to your hand. You can only use this effect of "Couplet the Melodious Songstress" once per turn.
Monster Effect
If this card is added to your hand, except by drawing it for your normal draw: You can reveal this card; Special Summon 1 Level 4 or lower "Melodious" monster from your hand or GY. If a "Melodious" Fusion Monster(s) is Special Summoned to your field, while this card is face-up in your Extra Deck (except during the Damage Step): You can place this card in your Pendulum Zone. You can only use each effect of "Couplet the Melodious Songstress" once per turn.