Learn Turkish: Doktorlar yeni bir hücre keşfetti. - The doctors discovered a new cell.

The verb is a combination of the noun keşif (discovery) and the helper verb etmek (to do/make). When they combine, the second vowel in keşif drops out, and the two words fuse together into keşfetmek. This is a common rule called vowel drop for certain loanwords.

Why is the word order yeni bir hücre rather than bir yeni hücre?

In Turkish, when a noun has both a descriptive adjective and the indefinite article bir (a/an), the article generally sits between the adjective and the noun. So, the structure for 'a new cell' must be adjective + bir + noun.

Since this lesson is about the passive voice suffix -Il, how would we change this sentence to say 'A new cell was discovered'?

You would remove the subject doktorlar and make the verb passive. To add the passive suffix -il to the consonant-ending root keşfet-, the 't' softens to a 'd' because of the incoming vowel, resulting in keşfedil-. With the past tense added, the full passive sentence becomes: Yeni bir hücre keşfedildi.

Why doesn't the direct object hücre take the accusative case ending to become hücreyi?

Because it is an indefinite direct object. The doctors discovered 'a' new cell, not 'the' new cell. In Turkish, you only use the accusative case suffix on specific, definite direct objects. Indefinite objects stay in their bare form.