If you make decisions about grammar based on what sounds right, you can blame the William Esty Ad Agency for any confusion over using like or as. Esty developed a slogan that became one of the top 10 jingles of the 20th century: Winston tastes good like a cigarette should.
The problem? Like is a preposition. Prepositions are useful little words that connect nouns, pronouns, or phrases to a sentence. Examples include of, to, and in, which rank among the ten words most commonly used in English. If you’re not sure whether a word is a preposition, try the squirrel test. Use the word to complete this sentence: The squirrel ran ________ . If the result makes sense, it’s probably a preposition: The squirrel ran after the bird, through the fence, up the tree, over the branch, across the wire, and into its nest.
Notice that none of the phrases that complete the squirrel sentence has a verb. A verb is a word that shows action or a state of being. Consider the phrase after the bird. If we ask, What did the bird do?, the answer is nothing. This group of words has no verb. Grammarians will not object if you use like to connect groups of words without a verb to a sentence; for example, My face burned like fire.
The problem, for purists, comes when like is used to connect a clause, a group of words that has a subject and a verb. In that case, the conjunction as should be used. Conjunctions are more powerful than prepositions; they can connect not only phrases, but clauses and whole sentences. In conversation, someone one might say Her jeans fit like a glove clings. However, in formal writing, as is preferred: Her jeans fit as a glove clings.
How can you tell whether you’re using like correctly? Look at the words that follow like. Are they just a word or phrase (a glove), or do they contain a subject and verb (a glove clings)? Use like to connect words or phrases; use as to connect clauses in formal writing.
For a more detailed explanation of prepositions, see “Prepositions for the Perplexed”: http://www.dailywritingtips.com/prepositions-for-the-perplexed/
“English 101: Conjunctions” explains different types of conjunctions: http://www.dailywritingtips.com/english-grammar-101-conjunctions/
Charles Darling explains the grammatical differences between like and such as: http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#like
A list of the top 10 advertising slogans of the 20th century is available at http://adage.com/article/special-report-the-advertising-century/ad-age-advertising-century-top-10-jingles/140154/
The 500 words most commonly used in English are listed on World-English: http://www.world-english.org/english500.htm
Jody Bruner summarizes the controversy over Winston’s use of like as a conjunction: http://brunerbiz.com/grammar/winston-tastes-good-like-a-cigarette-should/
Tina Blue explains why she thinks the distinction between like or as matters in formal writing: http://grammartips.homestead.com/like.html