The present perfect is used to express an action that started in the past and continues to the present. It can also be used to describe an action that happened in the past, but when that action happened isn't necessarily important.
The words "since" and "for" are often used with the present perfect in the first use of the present perfect just mentioned. For example, "I have been here SINCE 6:00." In this case, we know that the action is still continuing right now: the person has not left yet. He is still here. The word "since" tells us the exact time he arrived.
Here's another example with "since": "She has lived here since May." In this example, "since" tells us the exact month she arrived.
So "since" gives us an exact time an action in the past began. It could be a day, month, year, week, or time.
Sarah has walked this road every day since Tuesday.
They have eaten dinner at 7:00 p.m. since 1978.
Here is a bit of a twist with "since." You don't have to have an exact time, month, etc. after "since." You can have a clause with a subject and a verb. NOTICE, however that the verb in the clause after since MUST BE IN THE SIMPLE PAST, NOT in the PRESENT PERFECT.
Sarah has walked this road every day since she first arrived.
They have eaten dinner at 7:00 p.m. since Dad became ill.
Now, here are some examples and an explanation for using "for." "For" is used to give an AMOUNT of time since an action began.
For example, "I have been cooking dinner for three hours." "For" could also be used with "six years," "nine days," etc.
Here are some more examples:
Damien has read that book for seven days straight.
We have listened to this song for 20 days.
If you just know one word for the color "red," how can you express to the painter you've hired that you want to paint your walls "burgundy" instead of "fire engine red"? Likewise, how can you avoid using the same word in your essay for college over and over again?