4.He is dating another girl on the side. 他脚踏两条船.
可不要把这句说成 I have my feet in two boats,那可是典型的Chinglish。正确的说法应该是:He is dating another girl on the side. 或是He is dating another girl behind her back. (这个 her 指的是他女朋友)。
On the side 这句话你去餐厅用餐时也常用到,指的是主菜之外的副餐。比如说 I'd like salad with dressing on the side. 就是说“我要沙拉和沙拉酱”。和别人一起吃饭的时候,想问“要不要再点一份……”也可以用这个短语,比如:Would you like a baked potato on the side? (要不要再点一份烤马铃薯啊?)