Can anyone help me to understand the difference between "to bump your head on something" and "to bump your head against something". Is there any substantial difference or are they both used interchangeably? Thanks for your help.
Bump something against/on something are the same. Oxford Learner's Dictionary defines it in sense #2.
bump something (against/on something) - to hit something, especially a part of your body, against or on something
Kindly note that when we use against/on, the verb bump is used as a transitive verb. On the other hand, bump into/against uses the verb as an intransitive verb. Reference is on the same page.