This is the 5912th most frequent Turkish word.
testlerini
"Testlerini" means "their tests" in English.
In this sentence, 'testlerini' is in the plural form, and in accusative case, indicating that it is the direct object of the verb.
Öğrenciler testlerini tamamladı.
The students completed their tests.
Here, 'testlerini' refers to the tests prepared, showing possession (their tests) and is used with the suffix -ni for specificity.
Bu testlerini nasıl hazırladınız?
How did you prepare these tests?
'Testlerini' here implies possession (the tests of the subject, implied patient), and it appears in the accusative case as the object of the verb.