Questions & Answers about Bu ağaç geçen yıl öldü.
Bu means this when it comes before a noun. So bu ağaç means this tree.
By itself, bu can also mean this or this one. In contrast, Turkish often uses şu or o for that, depending on the situation.
Turkish does not have articles that work like English the and a/an.
So instead of saying something like the tree or a tree with a separate article, Turkish usually leaves the noun bare and lets context show the meaning. In this sentence, bu already makes the noun specific, so bu ağaç naturally means this tree.
Yıl means year.
Geçen comes from the verb geçmek, which means to pass. So geçen yıl literally means the year that passed, and this is the normal Turkish expression for last year.