if通常作为条件语句的一部分，用于在编程中实现逻辑分支。在if语句中，如果条件满足，则执行该代码块中的命令，否则跳过该命令并进入下一个命令。
if语句可以使用比较运算符，例如等于符号“==”、不等于符号“！=”、大于符号“>”等，来执行条件判断。
此外，if语句还可以和else语句一起使用，来处理非真条件的情况，使程序具有更完善的逻辑结构。
除此之外，if语句也可以使用嵌套的形式，将多个if语句组合在一起，实现更为复杂的逻辑控制。
百分之百的椰子汁,没有添加剂,没有糖。 if椰子水是真的纯椰子汁。if椰子水是产于泰国的一种天然无添加无糖的椰子水饮料，保留了椰子的原有味道，口感甘甜、清冽。椰子水也称为液体胚乳、植物牛奶，来自椰青中
表示条件，意思是如果；表示虚拟，意思是假如；表示是否的含义，和whether相同用法，连接宾语从句。
1.(表示条件)如果 ,主句用将来时,if从句用现在时表示将来
He will come if you invite him.如果你请他,他会来的.
2.(表示虚拟)假如,要是
时态：可以表示过去,现在和将来的情况.它的基本特点是时态退后.

a.同现在事实相反的假设.
句型 ：条件从句 主句
一般过去时 should( would) +动词原形
If they were here,they would help you.
b.表示于过去事实相反的假设.
句型：条件从句 主句
过去完成时 should(would) have+ 过去分词
If she had worked harder,she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday,he would have prevented me from going.
If he had come yesterday,I should / would have told him about it.
含义：He did not come yesterday,so I did not tell him about it.
If he had not been ill and missed many classes,he would have made greater progress.
含义：He was ill and missed many lessons,so he did not make greater progress.
c.表示对将来的假想
句型：条件从句 主句
一般过去时 should+ 动词原形
were+ 不定式 would + 动词原形
should+ 动词原形
If you succeeded,everything would be all right.
If you should succeed,everything would be all right.
If you were to succeed,everything would be all right.
3.是否 = whether 连接宾语从句
I wonder if she is ill.不知她是否病了.