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See this excellent blog for language news of the week, maintained by a teacher of A Level English Language.
Other useful links are collected here.
There are of course specific social reasons for particular changes, but some of the broad trends which have led to changes in the contemporary period (since 1950) are (in no particular order):
Can you think of more? A useful revision activity is to come up with examples of your own. You could then note as much detail as you can about these examples: How are they formed (blending, acronym etc)? Which word class(es) are they? Who uses them and in which contexts?
For older texts, think about: