on, upon
Generally interchangeable except in certain idiomatic expressions where upon is preferred:
looked upon
put upon, etc.
Upon takes a singular verb in expressions such as:
wave upon wave rolls upon the shore
day upon day goes ...
Generally interchangeable except in certain idiomatic expressions where upon is preferred:
looked upon
put upon, etc.
Upon takes a singular verb in expressions such as:
wave upon wave rolls upon the shore
day upon day goes ...