"THEN" and "THAN"

"Then" relates to time. "Than" is used for comparison. Ex.: First, we'll have lunch, and then we'll go for a walk. He is more experienced than I am.

"ALTOGETHER" and "ALL TOGETHER"

"Altogether" means completely or in total. "All together" means everyone or everything in a group. Ex.: The project was altogether successful. We sang the song all together.