Let us see how inversion should be used.
A reader from Bangalore has sent this e-mail : Could you please let me know on which day does your column appear in The New Indian Express and in which section of the paper?
If you read carefully, you will notice there is something wrong with the sentence. What is it?
The problem lies with the structure preceding and : on which day does your column appear. This, by itself, is correct. But when it is made part of a larger structure (as in the sentence cited) it becomes incorrect.
On which day does your column appear? This is a question and as required in questions, it shows inversion. The tense element in appear has moved to a pre-subject position. Since tense can't stand by itself, we have the carrier DO (Do + present tense = does).
But inversion in questions takes place only when the question is an independent one. More generally, inversion takes place only in main clauses. But in the reader's query on which day does your column appear we have a dependent question. It is part of the main clause : could you please let me know on which day. There is no inversion in subordinate clauses.
The sentence should read :
Questions which are part of a larger structure are indirect questions. They show no inversion.
Here are some more examples.
Supposing there is a question in the main clause and the sub-clause also has a question. What happens then? There will be inversion in the main clause. The sentence will have a question mark. But there will be no inversion in the sub-clause.
Do you know when she is coming?
A well-known feature of Indian English is the failure to observe inversion in questions.
One may understand the role of inversion in English and yet go wrong by not noting the restriction on it, a word or two may be in order about the meaning of inversion. In inversion the tense element in the verb is moved to a position in front of the subject. Since it cannot stand by itself DO is inserted and tense is shown on DO.
Question : Do + past he come : Did he come?
This is so with all lexical verbs : verbs expressing actions (physical, mental) speak, write, think, etc. But with the modals (shall, will, can etc) tense cannot be separated from them. So in question, the modal moves to the pre-subject position.
He can do what : What can he do?
There is a slight complication with BE and HAVE. These have non-finite forms : to be, to have. But when finite, tense cannot be separated from BE.
With HAVE, tense can stay with it or it can be moved, so we have :
Both are correct.