Vesting is another word for unlocking; the vesting schedule determines the quantity and frequency by which your stock options unlock.
Cliff is a short word for probation period. Upon maturity of the designated cliff period, you finally own the options that have vested to date. If you leave before the cliff expires, you will lose the right to pruchase the options that have vested to date.
Post termination exercise period (PTE) as known as the exercise window is the amount of time you have to buy your options once you leave. Yes, you need to BUY (4.) your options.
What is the total number of shares in the company right now?
What is the vesting/cliff period?
What is the post-termination exercise period?
Is there excess in the current option pool?
How much equity do you plan to sell by the end of the series A?
What is the current 409A price?