Do we hyphenate adjectives / numbers?
John Lawler calls this the eleven-year-old boy rule.
When a noun modifier consists of more than one word, it goes after the noun it modifies.
When a noun modifier consists of only one word, it goes before the noun it modifies.
Mnemonic: an eleven-year-old boy versus a boy eleven years old
A single word (which includes a hyphenated word, as here) is an adjective; a multi-word phrase is an adjectival phrase. Adjectives are not inflected for number, and will always be singular — hence "eleven-year". It doesn't matter whether you are talking about a seven-year itch or an eleven-year-old boy, it's always year before the noun. You might get an itch every seven years, where the adjectival phrase does have a plural.
In your example, six-year is a single adjective going before the noun it modifies. It should be hyphenated because it's a single word, and it should be singular because adjectives are always singular.
Note that whether the number is spelled out ("eleven") or presented in digits ("11"), everything else remains the same: year when before the main noun or years when after it; hyphenating before the noun and not hyphenated after it. The way the number is written is a matter of style and doesn't alter the rules about how the resultant single word or phrase is used.
The correct answer is this:
I have completed three years of a six-year course.
In this case, what you have is a compound noun. Compound nouns occur when you have one noun that describes another. Example: military campaign, job hunt, standing desk chair 5-year old.
When to use a hyphen:
You use the hyphen whenever there are numbers before the noun. Consider this:
- This girl is 6 years old and she hit her older brother.
- This 6-year-old girl hit her older brother.
Even when the noun is not descriptive, the numbers are combined.
- A 5-yead-old could do this.
Mistakes to avoid:
When forming a compound noun only the head noun (the final word) is given the plural "s". Example: Military campaigns (only campaigns takes the s) Cosmetics courses (in this case Cosmetics is always plural) Job seekers (only seekers takes an s)
I have completed three years of a six-years course?
In your example, "six-years" should not have an s