"May (might,can) could have" and "Must should have/should must have" impossible?

Question

My colleague and I were discussing Auxiliary Verbs and Modal Verbs when we came to a point where we started experimenting and we came to "May (might,can) could have" and "Must should have" whereupon we started arguing whether they are possible or not in English. While I'm saying they are, he states they are not. I would really like a native speaker's answer on this!

He's somewhat pointing to that two auxiliary verbs and modal verbs can't stand together. I doubt though that "can could (might, may) have" and "should must have" are possible so far.

For instance:

• If it weren't for the rain, the fire may (might) could have spread further.
• I could might (may) have (or might (may) could have) seen this movie, still I can't say for sure.
• She couldn't look straight into his eyes after all those lies. She must should have told him the truth. - I think even "must need have told him" or "should need to have told him" are possible.

So can we or can we not place two modal verbs or (and) auxiliary verbs together?

Answer 1

English auxiliary verbs combine into more complex constructions according to wholly inflexible rules: the sequence is always

• the modal component first (if it is present), with the following verb in its infinitive form
• the perfect component next (if it is present), using the auxiliary HAVE, with the following verb in its past participle form
• the progressive component next (if it is present), using the auxiliary BE, with the following verb in its present participle form
• the passive component next (if it is present), using the auxiliary BE, with the following verb in its past participle form
• the lexical verb is always the last.

Each construction is thus marked with a specific auxiliary verb, HAVE or BE, and there is a ‘ripple’ effect: the form (present or past participle or infinitive) of each verb is determined by the preceding component.

Note that the English 'modal' verbs can/could, may/might, must, shall/should, will/would are defective: they occur only in finite forms and have no non-finite forms (infinitives and participles).

Since a modal verb cannot be cast in the appropriate non-finite form, it cannot act as complement of a prior modal verb or auxiliary: except when two modals are conjoined (You can and should tell him), you can have only one modal verb in a clause, and it must be the finite first verb.

You occasionally hear paired modals in dialect speech, but this is emphatically non-standard and is generally taken to mark the speaker as uneducated.

The work-around for situations which call for "dual modalities" is to employ a periphrasis for the second modal:

*You might could do it → ^okYou might be able to do it.

A periphrasis like be able to VERB or need to VERB or be obliged to VERB in effect "restarts" the verb chain: a complex construction with its first element cast in the 'infinitive' may follow the to:

If you had planned better you might have been able to have been working offsite when he arrived.

In this case, however, the first element in the new chain cannot be a modal, because modals cannot be cast in the infinitive.

Answer 2

You can of course link an auxiliary and a modal auxiliary verb together in a sentence:

I could be sleeping instead.

You might have died!

And you can link the two auxiliary verbs have and be to form a perfect progressive tense (again, with a modal auxiliary if you want):

I could have been eating instead.

However, I can't think of any situations where two modal auxiliaries together would make sense, so I assume that this would be grammatically incorrect usage. If you want to emphasise an uncertainty you can use an adverb such as maybe, perhaps or possibly; if you wish to emphasise an imperative you can use an adverb such as absolutely or certainly.

I could possibly meet him.