我预定了星期四的课。 Subject+Predicate+(Attribute) Object

"I booked the lesson on Thursday" in this sentence, I (我) is the Subject, the Verb is" booked" and the Adverbial is on Thursday. Because the action happened in the past, Chinese verbs don't have changes in their tenses. But Chinese people usually add "了“ to show the action happened in the past. And we usually say ”on Thursday “ like ”在星期四" or "在周四"。 “在” means "on" and it is also a preposition. you can translate them like this: I (我) booked (预定了) the lesson（课） on Thursday (在星期四 or 在周四)。

我 预订了 上 星期四 的 课。

我订了周四的课！

If you write the English translation as this, you will know the answer: I booked the Thursday's class. 我预定了星期四的课。 The translation is one to one correspondence.