1.betterto 因为it is better for sb. to do sth某人最好做某事,固定搭配
2.must be sure to 等于是祈使句把主语放到前面来就是sb. must be sure to do sth.
希望我的回答能帮助到你O(∩_∩)O哈!有不懂的继续问我噢~答案是will surely ,您能给解释一下吗?这样也可以。调整一下语序就是:You will look afer the twins surely.suerly是副词,修饰look after,表示确定地,必须地,意思就是你必须照看一下双胞胎。希望帮助到你哈~