In the past I used to use "first come first serve"basis until I learnt that it should be "first come first served" basis from the Oxford Advanced Learner's Dictionary a couple of years ago. Grateful if any expert could kindly confirm which is correct and explain which one and why it is grammar error. Thank you!
The person who is the first to arrive, will be the person to be served first.
The person who arrives/comes first, will be served first.
First to come, first to be served.
First come, first served.