Q. When referring to a decade, do you use “was” or “were”? “The 1780s was [were?] an important period in history.”
A. Aside from certain quantities (“ninety dollars is a lot of money”), plural numbers usually take plural verbs: for example, “The ’80s were great!” To be perfectly correct, then, avoid the lure of the singular “period” in the predicate and use “were”: “The 1780s were an important period in history.” Compare “The decade of the 1780s was an important period in history.” See also CMOS 5.146 (on false attraction to the predicate noun).