A couple of times recently, I have seen disclaimers saying ‘Users of this car park do so at their own risk’. The meaning is clear, but this is an odd usage.
Why is that usage odd?
Usually, ‘do so’ is used only to replace a verbal phrase that repeats the same verbal phrase used earlier in the same sentence (or possibly in an earlier sentence). So, for example, sentence (1) is fine, as a concise version of sentence (2) or of the even more long-winded sentence (3).
(1) Customers who use this car park do so at their own risk.
(2) Customers who use this car park use it at their own risk.
(2) Customers who use this car park use this car park at their own risk.
Sentence (1) is fine because the verb use appears in the 1st half of the sentence.
Analysing the odd usage
But consider now the example above, repeated as sentence (4).
(4) Users of this car park do so at their own risk.
Sentence (4) is admirably concise. But in that sentence, no verb at all appears explicitly in the 1st half of the sentence. The verb is, of course, implicit in the first half because a user is someone who uses something. Indeed, if we decompose users into people-who-use, we get sentence (4a).
(4a) people-who-use [
of]this car park do so at their own risk.
In creating sentence (4a), I had to delete of. A noun needs that preposition as a link to the complement following the noun. But a verb doesn’t need of as a link to the complement following the verb (the verb’s direct object). Indeed, the verb cannot take of as a link to the verb’s complement.
Conclusion on sentence (4)
Sentence (4a) shows why sentence (4) is easy to understand.
But, to my ear at least, sentence (4) is completely ungrammatical (though also totally clear). ‘do so’ is possible only if it refers back to a noun that appears explicitly. It is not enough that the meaning of a preceding noun contains a component analysable as a verb that is only implicit (silent).
Why does do replace the repeated verb? The answer may not be clear from the examples given so far. That is because the verb in the 1st half of those sentences had no inflectional ending. So let me give 2 more examples.
(5) A customer who uses this car park does so at their own risk.
(6) A customer who used this car park did so at their own risk.
In sentences (5) and (6), the verb use is inflected to show it is:
- present tense, 3rd person singular [sentence (5]: use-s)
- past [sentence (6): use-d].
In essence, the verb do appears as a dummy auxiliary—one with no meaning. Its sole function is to be a vehicle carrying the inflectional suffix. (In examples (1) and (4), the inflectional suffix isn’t obvious because the suffix for all present tense forms (other than 3rd person singular) is zero.)
So, sentences (5) and (6) are perhaps best analysed as:
(5) A customer who use-s this car park doe-s so at their own risk.
(6) A customer who use-d this car park di-d so at their own risk.
(The verb do is slightly irregular so do-s surfaces as doe-s and do-d surfaces as di-d.)
Do support in other contexts
The construction that inserts do as a dummy verb carrying inflectons is often called ‘do support’. ‘Do support occurs in English in some other contexts, such as in negatives and in some questions.