I think this is the song with the longest title : How can you beleive
me when I tell you I love you when you know I been a liar all my life"
and that is all I can remember of said song.  Who wrote it? and who
sang it?  Does anyone know?

honey1 --

"How Could You Believe Me When I Said I Loved You When You Know I've
Been A Liar All My Life" was sung by Fred Astaire and Jane Powell in
the 1951 MGM movie, "Royal Wedding."

The song was written by Alan J. Lerner and Burton Lane, the
songwriting team that also wrote the Broadway hit, "On A  Clear Day." 
Lerner also paired with Frederic Loewe in writing the score to "My
Fair Lady."  A summary of Lerner's illustrious career can be found at
the following linked page:
Songwriters Hall Of Fame: Alan Jay Lerner  
http://www.songwritershalloffame.org/exhibit_bio.asp?exhibitId=42

Here is a link to the lyrics of the song, which includes notation of
which lines were sung by Astaire and which were sung by Powell:
FredAstaire.net
http://www.fredastaire.net/music/lyrics/howcouldyoubelieveme.htm

A recording of the Astaire/Powell performance is still in print as
part of a compilation of Fred Astaire's performances in a long series
of great MGM musical.  The All Music Guide listing of the recording
provides an interesting summary of Fred Astaire's MGM career:
All Music Guide: "Fred Astaire At MGM"
http://www.allmusic.com/cg/amg.dll?p=amg&uid=SEARCH&sql=A1tklu3ljan8k

A sample of the song is available at the Tower Records website:
Tower Records: Fred Astaire At MGM
http://www.towerrecords.com/product.asp?pfid=1190812&cc=USD

Facts about the movie, "Royal Wedding," can be found here at the All
Movie Guide website:
All Move Guide: "Royal Wedding"
http://www.towerrecords.com/product.asp?pfid=1190812&cc=USD

Interestingly, the cleverness of the title of the song and its
resemblance to humorous titles of old country songs has caused it to
find its way onto circulated lists of such country songs.  Here is an
example of such a list:
"The Worst Country Music Titles Ever Made"
http://members.aol.com/blueyedtom/rodeo/worsesongs.html

In fact, the song was written by two of the most sophisticated
songwriters in popular music history and was sung by the most urbane
performer in the golden era of movie musicals.


Google Search Terms:

"been a liar all my life"
Lerner Lane Loewe "Royal Wedding" "My Fair Lady"


If any of the above information is unclear, or if any of the links
don't work, please post a clarification request before rating this
answer.


markj-ga

Fantastic!  gives me information I wanted +