首先我们来看下it is、since和before的大致意思：
it is：词性为短语，it is是由it和is组成的短语，it代表无生命的物体或不明确的主语，is是be动词的第三人称单数形式。
since：词性为介词，since是一个基本介词，表示从某个特定时间开始，引导一个时间点、时间段或事件
before：词性为介词，before是一个基本介词，表示在某个时间点之前发生的事情
通过下面的表格我们了解下it is、since和before的含义、发音和用法
接下来让我们看下it is、since和before的用法区别：
1.含义：it is since强调从过去到现在的时间段，而it is before则强调在某个时间点之前
例子：
- It is three years since I graduated from university.
(It is three years since I graduated from university.)
- He left the house five minutes before I arrived.
(He left the house five minutes before I arrived.)
2.用法：it is since用于表示从过去到现在的时间段，而it is before用于表示某个时间点之前的动作
例子：
- It is two weeks since I last saw him.
(It is two weeks since I last saw him.)
- We should arrive at the airport two hours before the flight.
(We should arrive at the airport two hours before the flight.)
3.位置：it is since位于句子的开头，而it is before位于句子的中间或结尾
例子：
- Since I moved to this city, I have made many new friends.
(Since I moved to this city, I have made many new friends.)
- I should finish the report before the deadline.
(I should finish the report before the deadline.)
4.时间点：it is since后面跟的是起始时间点，而it is before后面跟的是结束时间点
例子：
- It is three o'clock since I last ate.
(It is three o'clock since I last ate.)
- You need to submit the application before June 1st.
(You need to submit the application before June 1st.)
5.动作延续：it is since强调动作从过去一直持续到现在，而it is before则只强调发生在某个时间点之前
例子：
- It is two years since I started learning piano.
(It is two years since I started learning piano.)
- They finished their homework before dinner.
(They finished their homework before dinner.)