Leave, if you like. Do you see anything odd or difficult about the syntax of that sentence? Because it does have an odd feature. Let me explain.
As a general rule, the verb like is strictly transitive: it must have a direct object.
Suppose you're trying to decide whether to buy an expensive new coat. It's not grammatical to say, *I'm not sure if I like.
Suppose someone asks you whether you get along well with a certain colleague. It's not grammatical to say, *Oh, yes, I really like.
The transitive verb like just cannot in general be grammatically used without an object. Yet in Leave, if you like, it doesn't have one.
It is true that there are certain syntactic contexts that can provide syntactic excuses for absence. Complements (such as objects) can be preposed, as in This, I like or This, I think you'll like. There the object is not so much missing as relocated to the beginning of the sentence for special discourse effect.
A variant of this is seen in Tell me who you like: this contains an open interrogative content clause beginning with who. The object is understood as a variable x that the question requests a (human) value for: it means "Identify for me a human being who is a value for x that will make ‘You like x’ true." The syntax of open interrogative content clauses requires a wh-phrase at the beginning and an absent noun phrase in some later position. So Tell me who you like is not a counterexample: there is an object, which is understood as an object (it denotes the entity that is the focus of the liking), but special syntactic conditions require that it not be overt.
Something very similar is true of You're the one that I like. The object is required to be non-overt here because of the syntax of integrated relative clauses: a relative clause in English is like a clause with a missing noun phrase, and optionally a that or a wh-phrase at the beginning.
But in Leave if you like, or closely similar sentences like You can go to sleep if you want, we do not have complement fronting or an open interrogative clause or a relative clause or any similar syntactic context providing a general excuse for absence of an object. There simply is no object.
What we have here, then, is a special construction with odd syntax that doesn't follow the rules that govern English syntax in general. It's out on its own. Sometimes I wonder just how many special constructions like this there are out there. Often Standard English syntax seems to me like a broad and apparently familiar landscape that is actually quite alien, only in ways that people typically don't notice. A strange syntactico-semantic Narnia that users of the language mostly don't realize they have walked through the back of their wardrobe into.
I am not sure yet [as of about 5 a.m. Eastern USA time] whether if you like is covered in The Cambridge Grammar of the English Language (it does, I confess, take a little time to check its 1860 pages; it seems to me that if this construction were covered it should have turned up somewhere in the special cases of object omission with transitive verbs in section 8.1.3 of chapter 4, pp. 300ff).
Update three hours later [about 8 a.m. Eastern USA time]: Aha! Thanks to Randy Alexander in Jilin City, China, the reference in CGEL has now been found. My gratitude to him. Although I do not claim that everything about English is covered, but most topics are covered somewhere in the book. What Randy found is that there is a reference on page 1529 regarding special ellipsis in complement clauses after conditional if. Other examples covered include We could make it Tuesday if you prefer __. It is noted that you get contrasts between conditional adjuncts and other kinds of adjunct; for example: Come if you want __ versus *Come because you want __.
I'm sorry I didn't recall that passage. Page 1529 is in chapter 17, on deixis and anaphora, which was mostly written by Lesley Stirling and Rodney Huddleston in Australia while I was teaching in Santa Cruz. It'll be a long time before I can say I have instant recall of all of the 1860 pages prepared by Rodney and me and our 13 additional collaborators. I can't hold all of CGEL in my head at once. I'm just glad that nearly everything, large or small, does seem to be in there somewhere.
By the way, those of you who are emailing me, don't think that putting this down as a special ellipsis construction associated with conditional clauses explains anything. It doesn't. It just notes the puzzle. Lots of people are writing to me to suggest paraphrases of Leave if you like with extra words added in various places ("Leave if you [would] like [to leave]", or "Leave if [that is what] you like", or "Leave if you [would] like [that]", etc.), as if somehow that made everything clear. This totally misses the point. Certainly, some sort of ellipsis is involved here, as CGEL says on page 1529. But ellipsis of what? Just anything you feel like leaving out? Why can't Leave if you like mean "Leave [only] if you [would not] like [anyone else to] leave [the party]"? You can't just supply additional words that would complete a meaning that you think is the right one, and call that an explanation!
And anyway, why is any ellipsis permitted? As noted above, the verb like doesn't normally allow its object to be omitted. It only happens in conditional adjuncts. Why would that be? I have no idea. You have no idea. None of us has an explanation. The comment on page 1529 of CGEL is in a shaded blue box headed "Some special cases". It might just as well have been in a gold chest emblazoned with "All you know is about to change". This is still a mysterious alien landscape.
Saying the magic words "There's no place like home" may get you out of Oz, but saying "Oh, it's just ellipsis!" doesn't spirit you away out of the syntactic Narnia that is English grammar.
Posted by Geoffrey K. Pullum at December 14, 2007 05:16 AM