'Hallo' means 'hello' 'Dag' is short for 'goedendag' (which means 'good day') and means 'bye'. In some cases, it could be used as 'hello' as well. This varies with different dialects. Personally, I'd advice you to only use 'dag' as 'bye' or when you're greeting someone you're passing by and don't intend on starting a (long) conversation with. For example, if you go hiking and greet another hiker. When you greet someone you intend to spend some time with (friends, family, co-workers) you'd best use 'hallo'.