As vs. Like: Clang, Clang, Clang Went the Conjunction

In this scene from Meet Me In St. Louis (circa 1944), Judy Garland's character says to her grandfather,

"It's our last dance in St. Louis. I feel like I'm going to cry!"

Sounds harmless enough. But for as long as anyone can remember, nothing has whipped grammarians into a lather like a good, old-fashioned debate about like and as.

Purists will tell you that like is a preposition and as is a conjunction, and that's the end of that. Meanwhile, other grammar experts say that it's perfectly fine to use like as a conjunction.

Why all the fuss? This is all really part of a much larger discussion about whether popular usage should influence proper, or standard, usage rules.

Quick rules:

• Like is a preposition. Use it to compare nouns and pronouns. In other words, there is a subject and an object. Joe throws like a girl. The woman in the photo looks like my mother.
• Use as or as if as a conjunction before clauses (containing a subject + verb). Joe throws as if he were a girl. It looks as if it's going to rain. I feel as if I'm going to throw up.
  
• Like can also be used as a conjunction before clauses. Joe throws like he is a girl. It looks like it's going to rain. I feel like I'm going to throw up.

Note that purists such as The AP Stylebook do not condone the third bulleted rule. Also consider this passage from Strunk & White's The Elements of Style (originally published 1959):

'The use of like for as has its defenders; they argue that any usage that achieves currency becomes valid automatically. This, they say, is the way the language is formed. It is and it isn't. An expression sometimes merely enjoys a vogue, much as an article of apparel does. Like has long been widely misused by the illiterate; lately it has been taken up by the knowing and the well-informed, who find it catchy, or liberating, and who use it as though they were slumming. If every word or device that achieved currency were immediately authenticated, simply on the ground of popularity, the language would be as chaotic as a ball game with no foul lines. For the student, perhaps the most useful thing to know about like is that most carefully edited publications regard its use before phrases and clauses as simple error.'

On the other hand, The Chicago Manual of Style says like can be used as a conjunction:

'Increasingly (but loosely) today in ordinary speech, like displaces as or as if as a conjunction to connect clauses. For example, in it happened just like I said it would happen, like should read as; and in you’re looking around like you’ve misplaced something, like should read as if. Because as and as if are conjunctions, they are followed by nouns in the nominative case {Do you work too hard, as I do?}. Although like as a conjunction has been considered nonstandard since the seventeenth century, today it is common in dialectal and colloquial usage {he ran like he was really scared}. Consider context and tone when deciding whether to impose standard English, as in the examples above.'

My take is that like and as/as if are interchangeable before a clause. It feels like I'm flying by the seat of my pants. It feels as if I'm flying by the seat of my pants. Both sound A-okay to me.

Some style guides, including The Chicago Manual of Style and Lynch's Grammar Guide, make the distinction between formal and informal writing. In other words, if you're writing a school essay or taking an exam on grammar, it's probably safer to think of like as a preposition and as as a conjunction.