Like I said...
Have you ever said the above? It's common (and perfectly tolerable) in informally spoken and written English; I'd expect to see the above construction in blog posts and emails, and to hear it in conversation. But in terms of proper grammar, as would be required in a formal research paper, the above is incorrect. The correct construction should be:
As I said...
The same problem occurs here:
You look like you've seen a ghost.
That should actually be:
You look as if you've seen a ghost.
[NB: Purists may see the above and argue that "as though" is more appropriate, claiming that "as if" is counterfactual. A question for another blog post, perhaps?]
"But, WHY?" I hear you screech. The general rule is this:
Use as (if) before a clause (or verbal construction); otherwise, use like.
CORRECT: You look like my sister. ("my sister" is a noun phrase, not a clause)
CORRECT: You look as if you're hungry. ("you're hungry" is a clause)
CORRECT: As I mentioned earlier... ("I mentioned" is a clause)
CORRECT: But in terms of proper grammar, as would be required in a formal research paper, the above is incorrect. (though not a clause, the phrase "would be required" is a verbal construction)
Simple? Clear? Try your hand at the following. Answers will be between the brackets following the quiz; highlight that space to see them.
1. Do _____ I do, not _____ I say.
2. Why can't you be more _____ your brother?
3. You've failed, Your Highness, for I am a Jedi, _____ my father before me.
4. _____ I was saying before I was rudely interrupted...
5. _____ sands through the hourglass, so are the days of our lives.
6. This case isn't _____ that case at all!
7. _____ you wish.
8. Please make corrections _____ necessary. (Watch out! This one's tricky.)
9. I am, _____ you now clearly see, a vampire.
10. All the same, I am not _____ other vampires, for I am a vegetarian.
ANSWERS (highlight the space between the brackets to see)
1. as, as ("I do" and "I say" are both clauses)
2. like ("your brother" is not a clause)
3. like (from "Return of the Jedi"; "my father" is not a clause)
4. As ("I was saying" is a clause)
5. Like (from an old daytime soap opera)
6. like ("that case" is not a clause)
7. As ("you wish" is a clause)
8. as (the sentence can be written out more fully as "Please make corrections AS THEY ARE necessary," or "...AS YOU DEEM necessary.")
10. like ("other vampires" is not a clause)