To convert Arabic numeral (Western numeral i.e. everyday numbers) you have to separate number into thousands, hundreds, tens and units. With Conventional Roman numerals you can express numbers from 1 to 3999. There are other alternative forms where you can express numbers greater than 3999.
Splitting 1986 we have: 1000 => M, 900 => CM, 80 => LXXX, 6 => VI. We have to concatenate corresponding roman numerals from thousands to units i.e. M, CM, LXXX, VI. So the final result is MCMLXXXVI
