Translation:My window is longer than yours.
Duolingo has certain standards and regulations which we must follow; one of which is that no changes can be made to the target language (Greek in this course) after it has been presented to the public. That's why we are creating a new tree to correct any errors that have occurred.
Each course has thousands of sentences each of which is created and entered letter by letter into the database. Unfortunately,errors occur. We are sorry and will try to avoid them in the future. If you have any suggestions for this sentence please post them.
The subject here is the use of double accents on one word in certain circumstances.
When a word is accented on the 3rd syllable from the end (the antepenult) e.g. δωμάτιο which is followed by a one syllable word which does not have an accent we add an extra accent to the main word to facilitate pronunciation. Note that the whole point is to show proper, smoother pronunciation. So, "my room" e.g. becomes "το δωμάτιό μου".
This is a very simplified explanation but I think it gets the idea across if you imagine pronouncing "δωμάτιο" + "μου" as two distinct words whereas the extra accent makes for a smooth sound. το δωμάτιό μου Ο δάσκαλό σου.
Hope that helps. In the present tree, many of these extra accents are missing but the new tree will be up to par. Well, that's what we're trying for.