The auxilliary verb ‘has’ is changed if the infinitive 'must' is used.Please quote the rule.

Question

Please refer the question to a grammarian or teacher of english.I need to know the rule. The answers I have received are either inferences or surmises.---NIRMALENDU

Answer 1

"Must" is not an infinitive.

Answer 2

This question is difficult to answer in a few words.

Here is a site with a page that will help you with the rules for auxiliary verbs, modals etc. There are examples and an optional quiz.

http://grammar.ccc.commnet.edu/GRAMMAR/a...