Learn Turkish: Çocuk aniden oyuncağı düşürdü. - The child suddenly dropped the toy.

The base verb is düşürmek (to drop). To make it past tense, we add the witnessed past suffix -DI. Because the last vowel in the verb stem düşür- is ü, the suffix follows 4-way vowel harmony and becomes -dü. Since the subject is third-person singular (the child), we don't need any extra personal suffix at the very end.

I learned that the word for toy is oyuncak. Why is it spelled oyuncağı here?

Two things are happening. First, we need the accusative suffix -ı because the toy is the specific direct object of the verb (it is 'the toy', not just 'a toy'). Second, when we add a suffix starting with a vowel to a word ending in k, the k softens into a soft g (ğ). This consonant mutation makes the word flow better!

Is the verb düşürdü related to the word düştü (fell)?

Yes! The base verb düşmek means to fall. By adding -ür, it becomes düşürmek, which means to cause to fall or to drop. So while düştü means something fell on its own, düşürdü means someone actively dropped it.

Could we put the word aniden (suddenly) at the very beginning of the sentence?