我都举例好了：第一类：Tom‘s coat is red,and his shoes are black.Tom的上衣是红色的，鞋子是黑色的（这里一般不用but）。It's cold outside,so you'd better put on your coat.外面很冷，所以你最好穿上衣服。Try it once more,though it's difficult.再试一遍吧，虽然很难。至于if，放中间不是不可以，只是一般不用加逗号：I'll tell you if I know it. 如果我知道，我会告诉你的I like mutton,but chicken is easier to cook.我喜欢羊肉，不过鸡肉更好...
