The 5 Core Rules
Rule 1, addition: When symbols are arranged from largest to smallest left to right, add their values. VI = 5 + 1 = 6. MCL = 1000 + 100 + 50 = 1150.
Rule 2, subtraction: When a smaller symbol appears directly before a larger one, subtract. IV = 4. IX = 9. XL = 40. CD = 400.
Rule 3, no more than three in a row: The same symbol can appear at most three times consecutively (except M). III is valid, IIII is not (use IV).
Rule 4, V, L, and D never repeat: VV, LL, DD do not exist. 10 is X, not VV.
Rule 5, only certain subtractions: Only I, X, and C can be subtractive. I can only precede V or X. X can only precede L or C. C can only precede D or M.