Qual a diferença entre ''on a/the seat'' e ''in a/the seat?

Há exemplos dos dois usos na internet, então quando usá-los?

TESTE DE NÍVEL
Faça um teste de inglês e descubra seu nível em 10 minutos! Este teste foi desenvolvido por professores experientes. O resultado sai na hora e com gabarito. Você ainda ganha o eBook sobre o Verbo Get em seu e-mail. INICIAR TESTE
4 respostas
  Resposta mais votada
7 61 291
He is in a seat. = He is seated in a seat/chair.

He is on a seat. = Meaning unclear.

The book is in a seat. = Meaning unclear.

The book is on a seat. = Someone has placed the book on a seat/chair.

He is in bed. = He is resting/sleeping or possibly even sick.

He is on a bed. = He is resting, but probably not sleeping and probably not covered with a blanket or sheet.

The book is in the bed. = Meaning unclear.

The book is on the bed. = Someone has placed the book on the bed (not under the covers).

He is in the toilet. = He is in the bathroom.

He is on the toilet. = He is in the middle of a bowel movement.
..................................................................................................
Without context, it can be difficult to understand some sentences. Even if a sentence sounds very strange, it may not be strange at all if we know the context. Probably 99% of the time, for example, I would tell you "Go sit in the chair." But maybe once in a hundred times, I would say "Go sit on that chair." Note that I would NEVER say "Go sit in the couch." On the couche, yes. In the couch, no. I think the example of the toilet is funny. Actually, "He is in the toilet" could mean that he fell inside the toilet. And it could also mean "He is in a lot of trouble, maybe with his wife or boss.
CENTENAS DE EXPRESSÕES DO INGLÊS
Preparamos um plano de estudos para ajudar você a dominar centenas de palavras e expressões do inglês em tempo recorde. EXPERIMENTE AGORA
7 61 291
a = um, indefinido
Find a chair. Nao importa qual, mas encontre uma cadeira.

the = o/a/os/as, definido
Find the chairs. Procure as cadeiras, as que ja foram mencionadas na conversa.
Desculpe-me, Thomas. Acho que me expressei mal. Gostaria, na verdade, de saber a diferença entre as preposições 'in' e 'on' antes da palavra 'seat'.

,
1 19
Parece que depende onde você vai se sentar. Olha um tópico que tem dúvida parecida com a sua no link abaixo:

http://www.english-test.net/forum/ftopic77523.html