Theory Progression Question

You are right. The song is in the key of F major and therefore the chords in that key would be F Gm Am Bb C Dm Em7b5 (note the chord should be written 'Bb' and not 'A#', will need a different thread to explain that one!) . However music is all about creating a picture of sound. Sometimes by adding notes or chords that are not in the key of the song give a distinctive colour or sound to the song. The Beatles did it to good effect as do a lot of other bands. Theory rules are there as a guide but are not set in stone so don't be afraid to step out and experiment, remember there are no 'Music Police' out there so do as you please and if it sounds good (to you) then play it!! Personally I love adding that major 3rd chord (as opposed to a minor) it really gives a lift in the song - but thats only my opinion!

In my experience it's very difficult to break the rules in music. If there is a melodic or a harmonic movement that occurs in a song, a pop song no less; then there is a virtual guarantee that someone, somewhere, in the four hundred years of development of our harmonic system has done it. And of course, some jerk will bust in and find or create a "theory" to justify why that movement works.

To answer your question: the A in that chord progression is a secondary dominant. I wasn't sure at first, but I checked a couple web sites.

Notes: dominant chords don't always need 7ths. One mistake there steve. The vii chord in a major key is only a m7b5 if you list the other chords as 7th chords. In this case it should be an Edim.