Sentence:  In the first of these sentences if by is the complement of come and Tuesday is an <WSD>adjunct</WSD> of time equivalent to on Tuesday , there will be strong stress on by in the spoken language ; but if a complement for come is implied and by Tuesday is a prepositional unit used as an adjunct , by will be unstressed or lightly stressed at most . 
Meaning of the word marked with <WSD> tags:  something added to another thing but not an essential part of it