Clarification of using topic/subject particles with demonstratives
What are the rules behind using demonstratives and the topic/subject particles in Korean? Are both the following correct:
이 편지는 맞지 않아요
이 편지가 밪지 않아요?
As far as I can tell, subject and topic particles would both translate to the same thing in English and just have subtle differences in Emphasis. The Duolingo course seems to (as a general rule), however, differentiate between them like this:
Bread (topic) is expensive
The bread is expensive.
There are a number of cases where it doesn't accept the other translation (e.g. translating to a general statement for the subject particle). Is this correct/an accurate reflection of their uses? Or are they literally always interchangeable depending on context?
And does this then extend to other areas, e.g. with the demonstratives (this/that)? Are there other contexts where there are specific rules or strict preferences of the topic subject over subject particles and vice versa?