G-d's name

QUESTION:
why do we say Adonoi instead of pronouncing Yud- Hey-Vav-Hey . Is it
written anywhere that we should not say G d's actual name

ANSWER:
Thank you for a great question. You raise a very interesting and important point.
The Talmud learns the verse (Exodus 3; 15)"This is my name forever" (Hebrew word for 'forever' is 'l'olam') as if it were written "This is my name to be hidden" ('l'helem'). From here they derive the principle that we should not pronounce G-d's name as it is written. The actual correct pronounciation of the name was only ever known by a few people in a generation, and only used in the Temple during services.
Nowadays we don't even know the correct pronounciation, and those who try to read it are making a mistake. They don't realise that the printers just put the vowels for 'Adonai' under the letters Yud-Heh to show how to read it (because ocasionally it is supposed to be read as 'Elohim' - in which case those vowels are used).
There is a deeper idea behind G-d's name being hidden. The act of creating the world was an act of G-d hiding Himself. It is only because He is hidden within creation that we can exist and function - if we would actually see G-d directly not only would we have no free choice, but we would not even exist. However, if He had hidden Himself too much it would have been impossible for us to 'find' Him or relate to Him. Therefore He can be found within the hidden. This is represented by the one Name which represents 'the essence of G-d' (as much as we can understand such a concept). All the other names of G-d are descriptions of His attributes, and all have meanings. The four letter name somehow represents what G-d is in his relationship with the world.
If you are interested you could have a look at a d'var Torah I wrote for last week's Torah portion (Bereishit) which touches on this idea of G-d being hidden. www.parshatshavua.blogspot.com
I hope this answer helps
Rabbi Sedley