English Grammar - Use of since and for

Use of since and for:

Since and for are two words that are used for similar, but not exactly the same, purposes.
Since:

• It gives the starting point of actions, events or states, continuing until the present.
• It refers to when things began.
• It can only be used with the perfect tenses.

Since + a point in time (in the past) until now.

• I've been waiting since 7 o'clock. 
• I have known him since January.
• I have been here since 5 o'clock and I am getting tired.
• It has been three years since the last earthquake.

For:

• We use For when we measure the duration
• When we say how long something lasts.
• We can use all verb tenses with For.

For + a period of time.

• I have known her for a long time.
• I lived here for ten years and then moved to another place.
• They are exercising for three hours today.

For vs. Since
Knowing when to use FOR and when to use SINCE is important.

FOR is used to mention a specific period (or duration) of time	SINCE is used to mention the starting point of a period of time (continues to now)
I have been a doctor for fifteen years. (duration = 15 years)	I have been a doctor since 1992. (starting point = 1992)