Present Perfect already & yet

Yet
*Use yet+ the present perfect in - and ? sentences to ask if something that you think is going to happen has or to say it hasn´t happened
* Put yet at the end of the sentence.
Have you finished your homework yet?
No, no yet. I haven´t finished yet.

Already.
*Use already in + senteces to say that something happened before now or earlier than expected.
*Put already before the main verb

Do you want to see this movie?
No, I´ve already seen it three times

Do you want a newspaper?
No, I´ve already bought one.


Already means that something happened earlier than we expected. With Present Perfect already usually goes after have or has and before the main verb.
Examples

   - We've already had our breakfast.

   - When are you going to do your homework?
   - But I've already done it!

   - Do you want a cup of coffee?
   - No, thanks. I've already had one."
Yet means that something that we expected has happened or hasn't happened. We usually put it at the end of a sentence.
Examples

   - Has the post arrived yet?

   - Have you done your homework?
   - Not yet.

   - Haven't you got ready yet? Look at the time!

Exercises:
http://www.eclecticenglish.com/grammar/PresentPerfect1C.html
http://perso.wanadoo.es/autoenglish/gr.presper.i.htm