1、简单的if语句只有一个测试条件和一个操作。代码示例如下：
age = 18
if age >= 18:
print("You are an adult now.")
1、if-else语句适用于只有两种情况的条件测试，当条件测试通过时执行一个操作，不通过时执行另外一个操作。代码示例如下：
age = 16
if age >= 18:
print("You are an adult now.")
else:
print("You will be an adult in " + str(18-age) + " years.")
1、if--elif-else语句适用于有3种情况的条件测试，依次判断每个条件测试是否通过，直到遇到通过了的条件测试。这个语句执行有先后顺序，而且当一个条件被满足时，接下来的条件测试会跳过，不再进行判断。代码示例如下：
age = 8
if age < 5:
price = 0
elif age < 18:
price = 10
else:
price = 20
print("The ticket price for you is "+ str(price) + ".")
1、if-elif-elif-else的语句类似if-elif-else的语句，不过是增加了判断条件。代码示例如下：
age = 68
if age < 5:
price = 0
elif age < 18:
price = 10
elif age > 65:
price = 0
else:
price = 20
print("The ticket price for you is "+ str(price) + ".")
2、if-elif-elif-elif的语句类似if-elif-else的语句，不过是将最后一个else改成了elif，功能相同，只是阅读起来可能更容易理解。代码示例如下：
age = 68
if age < 5:
price = 0
elif age < 18:
price = 10
elif age < 65:
price = 20
elif age >= 65:
price = 0
print("The ticket price for you is "+ str(price) + ".")
1、只有一个if的语句中，各个测试条件中只有一个条件满足要求。而在多个if的语句中，每个if语句的关系是并列的，不会受到其它if语句的影响。代码示例如下：
fruits = ["apple","peach","banana","orange"]
if "apple" in fruits:
print("She would like to eat some apples.")
if "banana" in fruits:
print("She would like to eat some bananas.")
if "strawberry" not in fruits:
print("She doesn't want any strawberry.")
print("That's all she want.")
