The present perfect continuous, the gerunds form of the present perfect and can be used with other past tenses like the simple past and the past continuous in discussions. It is used to communicate the idea of having started an activity in the past and carrying forward until now. The difference between this and its simple form is that there is a progression of activity from a past moment until now. That past moment can be specific or it can be unspecific.
In a sentence like she has been a member of the library for many years means that she began her membership and is still a member but she has been working as a librarian for 10 years means that for a period of ten years, she has been devoted to that task. The tense can be combined with another past tense to show a sequence of events such as: " He has been working since he was a young man." The use of since, yet and already is associated with this tense as with the simple form. In the aforementioned sentence there is a sense that from the time person was younger, he has worked.
Yet is used with the negative form as already is used with the affirmative form of the tense.
The use of yet can be equated with something up until now, so in a sentence like she hasn't come home yet it means that the person has not arrived home up until the time the person is speaking. Still is used as the affirmative form of yet so the same sentence is: "She still hasn"t arrived.'
Already is used to conform that an activity has been completed as in: "He has already handed in his report". This would be stated if there was any question about whether or not his report was received.
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