idiom · speaking with great fluency, skill, and persuasion, as if words were a flowing river

他在讲台上口若悬河，出色的口才赢得了全场阵阵掌声。

idiom · talking incessantly or with excessive ease; being glib or having the gift of the gab

那个推销员口若悬河地推销他的产品，根本不给别人插话的机会。