if so 与 if not 这是两个很有用的短语，常用来代替完整的句子。

• Have you got a free evening next week? If so , let's have dinner together.(=If you have, let's...)
  下周你能找个没事的晚上吗？如果能的话，咱们一块儿吃个饭吧。
• You may have some difficulty operating the machine at first. If so , do not hesitate to telephone our service department.
  你刚开始操作这台机器时可能会有些困难，如果这样的话，请马上给我们服务部打电话。
• Is anybody feeling cold? If not (If you're not feeling...), let's put the central heating off.
  有谁感到冷吗？如果没人感到冷，咱们就把中央供暖系统关了吧。