用碗把菜扣上免得凉了: cover the food with a bowl to keit from getting cold. detain
以便: so that; in order to; so as to; with the aim of; for the purpose of 短语和例子他们飞往那里以便能及时参加开幕典礼。 they flew there in order that they might be in time to attend the opening ceremony. 植物需要氮以便制造蛋白质
以便偷听: he ensconced himself in the closet in order to eavesdrop