The spelling of the adjective is hyphenated "multi-year."(Only Wiktionary shows a single word, unhyphenated form.)
Hyphenated
The word is spelled as one word "multitasking" but is occasionally seen hyphenated as in (multi-tasking).
No, "year long" is not hyphenated when used as an adverbial phrase, such as "The project will last year long." However, when used as a compound adjective before a noun, it should be hyphenated, as in "a year-long project."
Yes, it should be hyphenated like so: sixteen-year-old boys.
It should be hyphenated if used as a noun clause, but not if used as an adjective
Yes, "year after year" should be hyphenated when used as a compound adjective before a noun (e.g., year-after-year performance). However, it is not hyphenated when used as an adverbial phrase (e.g., they met year after year).
Yes, when used as an adjective. Year-to-year is hyphenated when used as an adjective: year-to-year comparison, year-to-year budget. Year to year is not hyphenated when it is used as a time period: We come back to this same beach year to year.
Four-year should be hyphenated but not institution.
Year-end is hyphenated in AP Style and is an adjective.
No it doesn't require to be hyphenated.
Yes it should be hyphenated.