I understand how to use these four expressions:
- Depuis que
- Il y a — in the temporal sense
- Il y a … que
I also understand the grammatical implications, but I do not understand the semantic differences between the four, and mainly between using depuis and il y a.
What is the semantic difference between these expressions? If examples could be given for the sake of providing context, semantic explanation, and perhaps even a general usage guideline (i.e. what type of practical situation warrants the use of one versus another), I would find this to be the most useful.