In this case, I would be more inclined to say that the person you are speaking to is someone who you are quite familiar with, i.e. have known for a few days. It sounds like you came to the same conclusion, because as you pointed out, the definite article is indicating a known context. The garden is described in such a way that implies that the person is already familiar with the garden. For example, you wouldn't say "the garden" unless it was already obvious to the person you are speaking to which garden you are referring to. Similarly, by saying "the apple tree" we can deduce that there is only one apple tree in the garden, but it makes more sense that this has already been explained to the person beforehand. This is also the reason why we always say "the sun(shine)", "the moon", "the sky", "the Earth" etc. in English, since there is only one of each (assuming we are talking about this planet, which we usually are). Sounds like you're getting a good feel for using indefinite/definite articles, keep it up!