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You can refer to the answer given by your friend as it is very detailed and comprehensible. Here is some additional information:

'Since' as a preposition means continuously from or counting from.

For instance,

• It has been raining since last night;

• She hasn't stopped crying since the time he left.

  It also talks about a time between the past and the present. For instance,

• There have been many changes since then;

• He has been dealing with many issues since he has become the MLA.