Since can be used as an adverb, a conjunction and a preposition.
Since as a preposition
As a preposition, since is followed by a noun/noun equivalent which acts as its object.
- She has changed a lot since her marriage.
- Everything has changed so much since our last meeting.
As an adverb
When since is used as an adverb, it is not followed by a noun.
- The boy went missing on Tuesday and hasn’t been seen since.
As a conjunction
Since can also be used as a conjunction. As a conjunction, since is used to connect two clauses. Clauses introduced by since typically show one of the two associations: time or cause/reason
- Since he had not studied hard, he failed his exam.
- Since I had no money, I couldn’t buy anything to eat.
- Since she was tired, she took some rest.
A since-clause introducing reason usually comes at the beginning of the sentence. A since-clause indicating time can come either before or after the main clause.
- She has written many books since she left college. / Since she left college, she has written many books.
When since is used to talk about time, the verb in the main clause is usually in the present perfect or past perfect (simple or continuous) tense.
- She had been acting in films since she was four.
- I have been working on this project since the end of June.
- It has been raining since morning.
Sometimes the simple present or simple past tense is also used.
Since means ‘starting at a particular point in the past and continuing until now’.
- I have known him since he was born.
- India has been an independent country since 1947.
- It has been several decades since India won a gold medal in Olympic Hockey.
- It has been several centuries since Vasco da Gama landed in India.
- She started teaching at twenty and has been doing it ever since.
- Ever since I was a child, I have wanted to become a writer.