I thought that, when a subject is given, with articles of clothing and parts of the body, "o/a" translates "his/her" in English?
No, it never means "his" or "her." It is implied, without context, that it is, though. Also, since for English it would be awkward without the "his" or "her," most times it is translated as such.
This seems inconsistent to me. There are times on here when similar constructions translate to a possessive (i.e. "The boy always forgets his coat") even without the possessive pronoun. This time, it says that is wrong...even though it is a more meaningful translation. What is the rule? How do we know when possession is and is not implied?
Very unusual sentence in English..."the boy allays forgets his jacket/coat" is more usual