Learn Turkish: Görünen o ki sen çok yorgunsun. - It appears that you are very tired.

Let's break it down piece by piece. Görünen is a substantivized participle of görünmek (to appear/seem), meaning "the thing that appears" or "what is apparent". O means "that" or "it", and ki is the conjunction "that". Literally, the phrase means "What is apparent is that...", which acts just like the English "It appears that...".

Could I just say Görünen ki sen çok yorgunsun without the o?

No, the o is required here. This structure uses o as a placeholder (or "dummy subject") that the ki clause then explains. You are essentially saying "The apparent thing is it (o), which is (ki)..." Without the o, the grammatical bridge between the participle and the explanatory clause is missing.

Since görünen is a participle (an adjective form), shouldn't it be görünen şey o ki?

You can add şey (thing), but it's completely unnecessary. Turkish frequently uses "substantivized" adjectives and participles, meaning they function as nouns all on their own. Görünen by itself perfectly translates to "the appearing thing" or "that which is apparent".

Why use the pronoun sen when the ending of yorgunsun already means "you"?