What are the prepositions for and since?
The prepositions for and since express the duration of an action or situation. For example:

    Paul: How long have you lived in Australia?
    Jen: I have lived here for one year.

    Nicole: How long have you worked downtown?
    Kevin: I have worked there since 2007.

Both answers explain how long the action or situation has continued.

Here we will look at for and since only with the present perfect tense. When used with the present perfect, for and since mean that the events haven't ended yet. So in the above examples, Jen is still in Australia and Kevin is still working downtown.

What is the sentence structure?
The structure is relatively easy. Always make sure the sentences use the present perfect, which consists of have + past participle. For example: I have lived in Australia for one year.

    S + V + O/C  |  for  |  period of time

    I have dated him  |  for  |  two months.
    He has lived at home  |  for  |  almost one year.

    S + V + O/C  |  since  |  starting point

    I have dated him  |  since  |  October.
    He has lived at home  |  since  |  last February.

In addition, the starting point may be expressed as a complete sentence in the past tense.

    I have dated him  |  since  |  I graduated high school.
    He has lived at home  |  since  |  he lost his job.

How are these prepositions used?
As previously stated, for and since express the duration of an action or situation. There is the added meaning that the action has continued up to now and the action will continue into the future. When you want to emphasize how long the action has continued, then use for.  When you want to emphasize when the action started, then use since. Let's look at the following:

    Hugh has studied medicine for three years.    
    This sentence focuses on the duration of Hugh's studies, which is three years. He began in 2005, is still there in 2008 (now), and will continue to be there for the foreseeable future.

    Hugh has studied medicine since 2005.
    This sentence focuses on when Hugh started his studies, which was in 2005. Because we know when he began medical school, we also know how long he has been there.

    Hugh has studied medicine since his mother died.
    This sentence also focuses on the start of Hugh's studies. The reference point is on an action rather than a date, and the description since his mother died provides additional information. Here the event (the death of Hugh's mom) may provide the reason for his medical studies.

Is there additional information on for and since?
Yes, there is one point. For and since both place emphasis on the duration of the action, and may be used with other perfect tenses. But when used with a perfect progressive tense, a little more emphasis is placed on the action than the duration.

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