Barry must have x has been relieved when he found

Hi guys, I have a simple question but that’s is annoying me too much.
I learned to use has been to he she and it, and have been to you, we, they.. But and when there is a hidden subject, what can I do?
For example..
Barry must have been relieved when he found his missing cellphone.
Barry for me can be replaced by he, so would be has been instead of have been in the phrase above.
The question is, why it’s have been instead of has been, specifically in this case.
Thanks for the help, have a nice day!!

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It's because of the "must". Whenever you use must,could,should and some others of the sort the verb following it stays in its neutral form (not really sure if "have" is called the neutral form of this verb or even if it's considered a verb at all for that matter but you get the point, don't you?). But if you're still in doubt and confused as to use which one between "have" and "has" you should use in these situations just use Portuguese to assist you. Ter = Have. Tem = has. Barry must have been... = Barry deve TER ficado/sido. Got it? One of my approaches to learning English has always been to not let Portuguese get in the way because more often than not it will make you confused but it does come in handy sometimes, if you can use it to your advantage like in this case then i'm all for it.
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