Like it or not, English is a complicated beast, based on many other languages. One of the most important of these is Latin.
Few people study Latin for GCSE, although it has recently increased in popularity. Most of us live our lives oblivious to it, and yet we use it every day without realising it!
Here is our essential Latin list of abbreviations:
a.m. & p.m., cf., e.g., et al., etc., i.e., N.B., P.S., Q.E.D., q.v., viz.
Here are some that are not abbreviated:
ad hoc, de facto, modus operandi, non sequitur, pro rata, quid pro quo, sic, vice versa.