Questions & Answers about Liseden beri mutlu değil çünkü yalnız.
Why does lise (high school) have the -den ending before beri?
The word beri (since) always requires the "from" ending (the ablative case) on the noun it follows. Since lise ends in a front vowel, it takes -den. Together, liseden beri translates to "since high school".
The translation says "hasn't been happy". Why doesn't mutlu değil use a past tense ending?
In English, we use the present perfect ("hasn't been") for things that started in the past and are still true now. Turkish simply uses the present tense! The word beri already tells us it started in the past, so the present-tense mutlu değil (is not happy) is all you need.
Where is the word for "he" in this sentence?
Turkish often leaves out subject pronouns like o (he/she/it). We know this sentence is about a third person because there are no personal endings (like "I" or "you" suffixes) attached to değil or yalnız.
Can yalnız mean "alone" as well as "lonely"?
Yes, yalnız can mean both physically being "alone" and the emotional feeling of being "lonely". Context usually tells you which one fits best, though here both make perfect sense as a reason for not being happy.