When you are talking about an event or situation in the past, you use since to indicate that another event happened at some point later in time. About six thousand people were arrested, several hundred of whom have since been released
From a definite past time until now; as, he went a month ago, and I have not seen him since
You use since to mention a time or event in the past when you are describing an event or situation that has happened after that time. The percentage increase in reported crime in England and Wales this year is the highest since the war He turned out to have more battles with the Congress than any president since Andrew Johnson. Since is also a conjunction. So much has changed in the sport since I was a teenager Since I have become a mother, the sound of children's voices has lost its charm
If you say that something has long since happened, you mean that it happened a long time ago. Even though her parents have long since died, she still talks about them in the present tense