Thats a great question that im by no means qualified to answer, but I think that wash can go for more than rinsing i dont think its literal like I could picture a mother scolding her child because he didnt wipe his apple off after he picked it in an orchard, and this sentence is her speaking to the childs father when he asked why she scolded the child. So I think if they were implying rinsing or cleaning that they could use another more specific word. At least those are my thoughts.
Regarding the position of the nicht, it's definitely possible and would be the way to go if you want to emphasize that he is washing something different:
„Er wäscht nicht seine Äpfel sondern meine.“ – „He's not washing his apples but mine.“
„Er wäscht nicht seine Äpfel sondern seine Birnen.“ – „He's not washing his apples but his pears.“
But Äpfel is plural, Apfel is singular:
nominative: „Sein Apfel schmeckt gut.“ – „His apple tastes good.“ / „Seine Äpfel schmecken gut.“ – „His apples taste good.“
accusative: „Er wäscht seinen Apfel.“ – „He's washing his apple.“ / „Er wäscht seine Äpfel.“ – „He's washing his apples.“
I type this the 2nd time. The computer changes my typing and yes, I do lose hearts because of it. It keeps on changing NICHT to niche. If you say Er wäscht nicht….. Then you are saying he does not wash anything. Ich sehe die Kuhe nicht versus Ich sehe nicht die Kuhe. If I was a blind person, I would say ich sehe nicht die Kuhe. But I do have a sense of sight. I just cannot see the cows right now, so I say, Ich sehe die Kuhe nichtttt.
It*s reasonable to assume that there are two people but not required. However, in the context of „He is washing someone else's apple.“ the sentence would be „Er wäscht nicht seinen Apfel.“
In the context of „He is not washing his apple. Her sister however is washing hers.“ it's appropriate. Or with „He is not washing his apple. Someone else is washing his apple for him.“
Am I the only one who hears mischt instead of wäscht during the normal speed audio recording? When I play the slow recording, I can hear it okay, but the fast one sounds like mischt to me (whether or not mischt is even a word is beside the point), even after listening to it several times.
It is possible, assuming you don't use a very obscure computer: http://en.wikipedia.org/wiki/Umlaut_(diacritic)#Computer_usage
seinen is an inflection of the possessive pronoun/determiner, translating to his (or its for a neuter possessor)
ihn is the accusative case („direct object case“) inflection of the personal pronoun er, translating to him or it for a neuter noun.
„Er wäscht seinen Apfel.“ – „He's washing his apple.“
„Er wäscht ihn.“ – „He's washing it.“
The stem depends on the gender and number of the possessor:
3rd person singular masculine possessor: sein-
3rd p. s. feminine possessor: ihr-
The ending depends on the grammatical case and the gender and number of the possessed items:
accusative singular masculine (as in „den Apfel“): seinen Apfel
accusative singular feminine: seine Birne
You can look up all stems and endings here
See my comment here: http://www.nthuleen.com/teach/grammar/adjektivendungenexpl.html
In the sense, perhaps, that everyone else is in the process of washing their apples, and I've just seen him walk straight past the sink and take a bite of his apple, yes.
But it's still in the present tense that he's not washing his apple, so perhaps I'm entirely sure what you're asking.