Take this entry on noun plurals, from,of all books, TheNew York Times Manual of Style and Usage:
Nouns derived from foreign languages form plurals in different ways. Some use the original, foreign plurals: alumnae; alumni; data; media; phenomena. But form the plurals of others simply by adding s:curriculums; formulas; memorandums; stadiums.
Were you stopped short, as I was, by “form the plurals”? The passage begins with two indicative sentences whose subjects refer to foreign nouns, and whose predicates comment on how those nouns “form” or “use” the plural. Then, without warning, the third sentence shifts to the imperative mood and we are supposed to have the reader,not the nouns, forming plurals in a certain way.