The most common use of apostrophes in English is for contractions, where a noun or pronoun and a verb combine. Remember that the apostrophe is often replacing a letter that has been dropped. It is placed where the missing letter would be in that case.
The apostrophe has two functions: it marks possession, and it is used in contractions to indicate the place where the letters have been omitted. In singular, possession is marked by 's, written immediately after the possessor. Important: there is no apostrophe before the possessive –s with pronouns.
The store gave out samples by the dozen. - Is the correct phrase, as an adverbial phrase is a word/ phrase that modifies the/ qualifies an adjective. So the 'dozen' modifies the sentence.
