Learn Turkish: Kedi dün öldü. - The cat died yesterday.

The past tense suffix follows the 4-way vowel harmony rule. The verb root is öl (to die). Because the vowel in this root is ö, the suffix changes to -dü to match. If the root had an e or i, it would be -di.

Does öldü need an extra ending to show that 'it' (the cat) died?

No, it doesn't! In Turkish, the third person singular (he, she, or it) does not take an extra personal suffix in the past tense. The verb root plus the past tense marker (öl + -dü) is all you need to say 'it died'.

How do we know it means 'The cat' and not 'A cat'?

Turkish doesn't have a direct word for 'the'. When a noun is the subject of the sentence and doesn't have bir (a/an) in front of it, it is naturally understood as specific. If you wanted to say 'A cat died', you would add it and say Bir kedi dün öldü.

Could I change the word order and say Dün kedi öldü?

Yes, you can! Time words like dün (yesterday) are quite flexible. Putting dün right before the verb just puts a little more emphasis on exactly when the event happened, but putting it at the beginning is also perfectly correct and natural.