I haven't gotten achievements for finishing missions in Halo 3?

[Kevin]

The achievement guide says that you get achievements, 20G for every mission you complete. But I can't figure out when the missions end. I have went through several pauses where the screen goes widescreen and in the lower right it says a name of something. I just got Halo 3 for Christmas and started playing it a couple days ago. But why haven't I been getting my achievements?
How do I change the difficulty?

 

[macdady]

you have to play on normal level or higher to get hte acheivments

 

[Bijan]

1. Halo 3: ODST or Halo 3?
2. For Halo 3, start a new campaign.
It must be in NORMAL, HEROIC, or LEGENDARY
Easy doesn't give you anything.