OK: here are five worksheets on the homophones vain and vein, which are, respectively, a noun and an adjective. Remember that the noun for vain, in the sense of “having or showing undue pride in one’s appearance or achievements” is vanity. And when one uses (as the exercises show) vain as an adjective to mean “marked by futility or ineffectualness” the preposition in generally precedes the adjective (i.e. in vain).
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Mark's Text Terminal 