I have come to the conclusion that to the beginner it is useful to equate the particle は with the article ‘the’ in English. Even more so for a Swedish speaker since the Swedish article is an ending.
A difference is that we say あなたは but not “the you”. However あなたは is short for an old あの方 and not a true pronoun. Similar ways of avoiding a true ‘thou’ were common in Sweden 50 years ago (and forced an article). English examples are rarer but we can say “the other side” instead of “they” when referring to another political party.
Thus I propose は as the definite form nominative case ending and が as the indefinite form nominative case ending.
On the whole I think that the distinction between Japanese postpositions and endings in Germanic languages is misleading. Why is の a postposition and ‘s an ending? Presently I see は, が, の and を as case endings and I would appreciate to see examples where it does not work out.