"In the winter I am unhappy, in the summer I am happy."
Translation:Kışın mutsuzum, yazın mutluyum.
These sort of suffixes are mostly outdated and used only in certain phrases for example:
Kışın: in winter Yazın: in summer Güzün in fall (literary/poetic/not used in everyday language) Gündüzün: during the daylight (outdated, possibly used in some outer provinces) Öğleyin: at noon (mostly outdated, but still used none the less) Akşamleyin: at evening Geceleyin: at night Sabahleyin: at morning
as you can see they are used to indicate a time period, such as:
Yazın tatile gideriz = We go for a vacation in summer
This states generalization and it's basically the same as: Yazları tatile gideriz
Sometimes this can also imply desire/wish. But that depends on the context and it usually refers to the upcoming summer, something roughly like: "Let's take a vacation in (this) summer".
It is not completely "wrong" but the suffixes already give that meaning, so it feels redundant. You can maybe put "ben" or another subject (with the proper suffixes on the verb according to the subject used of course) at the beginning of the language if you want to emphasize on that subject.
Actually this sentence is merely indicating a state, so there inclusion of ben would change the meaning to a emphasis on ben, like being happy in the summers only applies for you. In most cases, just the suffixes are enough to add subject to a sentence in daily usage.
But to answer your question, it would be:
"Ben kisin mutsuzum, ben yazin mutluyum.
This is semantically not wrong but awkward to use. If you want to really accentuate the "ben" then it is:
"Kisin BEN mutsuzum, yazin BEN mutluyum"
Both of these are for very special occasions though. Not really necessary.
I think it is due to the word "kisin"in your sentence. It must be "kışın". Always if a word could have another signification Duolingo is not accepting writing mistakes. I think it is given wrong because there exists the word "kişi" = person and what you wrote is nearer to the genitive form of kişi (kişin) as to kışın. That is only what I am thinking.