Mortgage Banking question – paying additional principal

I have a question for someone familiar with mortgages.

In my shiny new mortgage payment book, the coupons have an entry for paying additional principal.

If I pay additional principal each month, is the life of the loan shortened, or is the loan recalculated for a lower payment for the original term of the loan?

The life is shortened.

If you make one extra payment at the end of the first year, a 30-yr loan will be reduced by over 1 year (if I did my calc right).

The payments never change (with exception of variable rate) no matter what happens. Miss a payment even if you have paid a large sum towards the principle and you have missed a payment. Variable loans only vary according to the interest rate and have nothing to do with the amount you have paid in.