How to accompany on the harmonium

I play the harmonium when I accompany a singer. I asked my teacher several months ago as to what specifics to follow when accompanying.

He said for classical music, I should play exact note for note what the singer is singing. I had to also play right a the singer was singing, with no delay, as that would make it sound off key. I also had to refrain from playing ahead of the singer. All of this was difficult, because I seemed to have a habit of lagging behind if I didn't know the song, or playing too fast if I did. Through experience, I learned that I had to pump a lot of air into the bellows and play very loud so that the singer could hear over his own voice. I am still kind of confused as to how to do that and still be able to listen to what he is singing. 

For light music, My teacher taught me to use chords in the song as often as possible. This is difficult, because I have to remember the chord shape in time, and not play it too long such that the song had already progressed. I presume that that skill will come with experience. In general, accompaniment is easier for classical music.