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What is the difference in meaning between these sentences?
John has written two essays this morning.
John has been writing an essay all morning.
The police sergeant has interviewed two people so far today.
The detectives have been interviewing people all week.
(1) John has written two essays this morning. So, this means that the essays are finished.
John has been writing an essay all morning. This means that it is still in progress.
You should be able to expand for the other examples.
The present perfect continuous means that it has been happening over a period of time, often a specified time (either in the same sentence, or in further context - especially where the present perfect continuous is used in answer to a question). It usually implies that it is still happening, though it is sometimes used where it has just very recently finished.
The present perfect is for things that happened in the past and are complete, or have ceased happening. Depending on the verb and the object, it may strongly indicate that the task is complete, as in "interviewed two people" or "written two essays".
In addition to the other answers: with "how much" and "how many", the present perfect tense is used. "How long" is more common with the present perfect progressive.
