Posse is the infinitive of an irregular Latin verb meaning "to be able". It can also be used as a noun meaning "ability", "power", "potentiality". Two common phrases that use posse this way are
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Possum is an English loan word in Italian.
Specifically, the word is a masculine noun whose singular and plural forms are the same. It refers to the Australian marsupial in the Phalangeriformes suborder in the mammal class of the animal kingdom. The pronunciation will be "POS-suhm" or "POS-soom" depending upon the speaker's birthplace.
It is none. It is an irregular verb, so therefore you will have to memorize the endings and principle parts separately, as for all irregular verbs such as sum, esse and nolo, nolle.