According to this Website it is ok to use the auxiliary verb in the same sentence that uses the modal could but it is marked as wrong to use it in another sentence which uses a modal as well.

I decided to do further research and I got this:

The sentence 'Can you tell me how often the buses run?' is correct and can be used in written English. You can use it when asking someone, either in person or in writing, to provide information about the frequency of a bus service. For example, "I'm planning to take the bus downtown. Can you tell me how often the buses run?". https://app.ludwig.guru/s/could+you+tell+me+how+much+does+a+taxi+cost

Could some one explain why? I am aware that one shouldn't combine the auxiliary verb do with modals but it drew my attention that this search engine says otherwise?


3 Answers 3


I think that you are confused about when various auxiliary verbs can be used together. There is no prohibition against using multiple such verbs in a single sentence. For example:

I could go to the movies, but I do not want to.

However, the modal auxiliary verbs and the auxiliary verb "do" have the following restriction: They must always head predicates. That means that a modal verb will generally not appear in the same predicate as auxiliary "do".

One exception is that a predicate may contain another one, e.g.: "You must know that we do want you to succeed." Another is that the auxiliary verbs may be in series (though the acceptability of this will vary), e.g.: "We can and do go swimming every week."

(By the way, the examples on sentencestack.com appear to have other problems. I recommend using a better site to learn English.)


There is a discrepancy between the answers given by the website; there should not be any difference in the two cases, between the use of could and can.

Both subordinate question-clauses should be in the statement form (SVO), as shown below:

Could you tell me how much a taxi [ride] costs?

Can you tell me how often the buses run?


OK: Could you tell me [question form] how much a taxi costs [declarative statement]

And this: Could you tell me how much a taxi does cost? Is only right in one environment, as follows:

Person 1: Oh no, that's not the price. It does not cost that!
Person 2: Really? Then, can you tell me how much a taxi does cost?

In the sentence above Person 2 is clarifying a statement by Person 1 but it would not used except in a context like that.

  • Did you mean "it would not be used "
    – Quique
    Sep 16, 2023 at 16:51
  • @Quique Yes, it would not be used EXCEPT in a context like that. OR: It would be used in a context like that. Same thing.
    – Lambie
    Sep 16, 2023 at 16:56
  • I have already deleted the screenshots and posted the links to the website as you asked, might you check them please?
    – Quique
    Sep 16, 2023 at 17:31
  • @Quique It's ok but next time just take the sentences and get URL from the top. :)
    – Lambie
    Sep 16, 2023 at 18:05
  • @Quique Looks OK to me. :)
    – Lambie
    Sep 19, 2023 at 14:25

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