'wenn' means 'if' ( or when), and 'falls' means 'only if'. This means the condition in the if-sentence is much stronger, for example: Wenn/Falls es regnet, gehe ich nicht spazieren. In the wenn-sentence, the rain is one condition (among others). In the falls-sentence, it is the only condition. In general, you can always use 'wenn', but not always 'falls'.

my teacher says that 'falls'can only used on the'Hauptsatz' .But mit 'wenn' it can also used on 'Nebensatz'