# if vs. whether (complementizer)

I'm studying syntax with 'Introduction to government and binding theory' by Haegeman by myself and I encountered something I don't understand.

According to PRO theorem, PRO must be ungoverned. And these two sentences were presented.

(a) *John doesn't know [if [PRO to leave]].
(b) John doesn't know [whether [PRO to leave]].

And he explains that in (a), if is the head and it governs PRO so the sentence is ungrammatical. But in (b), whether is at the [Spec, CP] position so nothing governs PRO and the sentence is grammatical.

As I understood earlier, if and whether are samely complementizer. But how come if goes in [C, CP] while whether goes in [Spec, CP]? Why these two act differently and how can I tell it?

I'd appreciate it if someone could help me. Thanks! (If I posted this on the wrong place, please let me know. I'm not sure if I should go to English forum.)

• Since non-terminal nodes never appear in the data, you can put whatever words you like in them. In this case, arranging things the suggested way makes this difference (whether can govern an infinitive VP, but if can't) seem to fall out naturally, instead of being a random chunk of difference between two lexemes. – jlawler Jul 12 '17 at 17:34
• The query pro-forms have more functions than simple subordination. 'Whether' is a query pro-form for 'either', with the other choice (to stay) elided in your example. – amI Jul 12 '17 at 21:42
• In (b) "whether" is an interrogative subordinator introducing an interrogative infinitival. The meaning is deontic, as if the modal "should" were included: John doesn't know whether he should leave. By contrast, the subordinator "if" cannot introduce interrogative infinitives -- only subordinate content clauses (embedded questions) which (a) is not. Which explains why it is ungrammatical. – BillJ Jul 15 '17 at 16:07