A smaller number in front of a larger number means subtraction, all else means addition. For example, IV means 4, VI means 6.

You would not put more than one smaller number in front of a larger number to subtract. For example, IIV would not mean 3.

You must separate ones, tens, hundreds, and thousands as separate items. That means that 99 is XCIX, 90 + 9, but never should be written as IC. Similarly, 999 cannot be IM and 1999 cannot be MIM.

I The numeral one. II is two, III is three. You seldom see IIII as 4, since IV can also mean 4, plus its shorter to write.

V The numeral 5. IV is 4, VI is 6, VII is 7, VIII is 8.

X The numeral 10. IX is 9, XI is 11, etc.

L The numeral 50. XL would be 40.

C The numeral 100. Think of Century having a hundred years. C is short for the Latin word Centum, but that's not very easy to remember.

D The numeral 500.

M The numeral 1000.

Sometimes you will see a numeral with a line over it. That means to multiply it by 1000. A numeral V with a line over it means 5000.