Writing 1986 in Roman numerals would look like this: MCMLXXXVI. Note that when writing in Roman numerals, if a smaller number comes before a larger number, it is subtracted from it.
The online chart from Kent State University is handy when needing to translate the numerals into numbers and vice versa. Since the numeral C comes before the second M in this instance, we read it using subtracted notation, meaning that the C is specifically placed before the M to subtract it's value, 100 from 1,000. Subtracted notation is used in Roman numerals to prevent four similar numerals from being repeated in succession.