WH- TO

A WH- TO complement is like a Modal (FOR) TO clause with a wh- word (as in a WH- complement) at the beginning, in place of for. The complement clause subject must be coreferential with either main clause subject or object, and omitted. The wh- word refers to any constituent of the main clause except its subject.

A WH- TO clause combines the meanings of Modal (FOR) TO and of WH-complements. It refers to some activity in which the subject has the potential for getting involved, and it is an activity about which some clarification is required. Compare the THAT construction in (54a), the Modal (FOR) TO one in (54b), the plain WH- in (54c), and the WH- TO complement in (54d).

(54a) He knew (that) he should stand up when the judge entered

(54b) He knew to stand up when the judge entered

(54c) He didn’t know whether he should stand up when the judge entered

(54d) He didn’t know whether to stand up when the judge entered

Any verb which permits both a WH- and a Modal (FOR) TO clause will also take a WH- TO complement. Besides know, these include remember and decide, e.g. He couldn’t remember whether to close the window or leave it open, and I’ll decide tomorrow who to fire, when to fire them, and how to appease the union. It appears that no verb which does not take a plain WH-complement may take WH- TO. But there are verbs which take WH- and not Modal (FOR) TO and do accept WH- TO, e.g. report, think (about) and understand, as in She’s thinking about whether to accept the offer, and I don’t understand how to behave.

A plain WH- complement, such as (54c), may include whether or if, with a subtle difference in meaning (see Bolinger 1978); only whether is allowed in a WH- TO clause such as (54d). All other wh- words may begin a WH- TO complement excepting why. There appears to be a semantic reason for this. Consider first

(55) I don’t know why I should go

This sentence is acceptable, with a meaning something like ‘I don’t know what my obligation is supposed to be that would make me go’. Now recall that there is always a semantic difference between a THAT construction including should and a Modal (FOR) TO construction. Whereas a THAT-plus-should clause refers to some extraneous obligation, a Modal (FOR) TO construction refers to the potentiality of the subject’s getting involved in the activity. *I don’t know why to go is ungrammatical since why, demanding clarification of the reason for entering into an activity, is semantically incompatible with Modal (FOR) TO, stating that the subject does volitionally become involved in the activity. But Modal (FOR) TO is perfectly compatible with all other wh- words, e.g. I don’t know how to open the door/when to arrive/who to blame.