Must have been OR must has been?

0No, much the same way you wouldn't use the first person singular below:02br
00She must have her gloves.02br
00She will have her gloves.02br
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00But you must use 'has' in these cases:02br
00The puma has been in the possession of a private collector.02br
00She has gloves.0-

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00Hi Warrener, Your question......02br
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00>>Should we use "must has been" in this sentence, since "puma" is NOT a plural?02br
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00I am afraid not. 00 00Verbs used after modal ( and auxiliary)words must remain in their basic form. (Must, shall, will, can ought to, etc…)02br
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0 After 01i00can, could, will, would, shall, should, may, might, 02i00and01i00 must02i00, the only form of 01i00have02i00 which is possible is 01i00have02i00 itself, 01u00never02u00 01i00has02i00 or 01i00had02i00. (The word 01i0

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01cite10Warrener12cite10There is a confusing sentence:12br
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10"The experts from the London Zoo thought that this puma must have been in the possession of a private collector and somehow managed to escape."12br
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10Should we use "must has been" in this sentence, since "puma" is NOT a plural?

you cant say must has been, cause modals like must, can , could , might .....etc take base form of verb after it so the base form of has and had is have (must have)

Bare Infinitive unmarked form of the verb (no indication of tense, mood, person, or aspect) without the particle "to"; typically used after modal auxiliary verbs; see also infinitive eg: "He should come", "I can swim"