'Is de appel groen?' (Is the apple green?) is a question, which can simply be answered by saying 'yes' or 'no'. 'De appel is groen?' (The apple is green?) is mainly a statement of surprise ('De appel is groen? Iemand anders zei net dat hij rood is.' (Someone else just now said it is red)) or, for example, when you say that the apple is red and your friend says 'The apple is green? Why can't you see that it is so very obviously green?'
To return to your question: 'De appel is groen?' is a statement of surprise, so the translation would be 'The apple is green?'. To have the translation 'Is the apple green?', the sentence should be the question 'Is de appel groen?'. Hope this was clear.
However, since this is an advanced distinction (and I don't think I've seen it made in any of my other languages here in this way....), I would suggest adding a context marker, like, De appel is groen? Really?
shouldn't it be 'is de appel groen'?, since the verb occupies the first position in a phrase when it's a question...?
"De appel is groen?" is simply a statement turned into a question, which is also possible in English when saying "The apple is green?"
But I thought always inversion..?
Hij ziet mij. Ziet hij mij?
Does this mean then that Hij ziet mij? is a possibility?
In addition to KaiEngle: 'Hij ziet mij?' (a statement) is (only) used when expressing surprise, like in 'oh, seriously? I didn't notice that'. As a question, you always use 'Is de appel groen?'.
So then this is also the case with "De appel is groen?" ? As a way to express surprise? "De appel is groen? Ik dacht dat het is rood."
Yes. Small correction: 'Ik dacht dat het is rood' should be 'Ik dacht dat hij rood is'.