Word order in Russian is sensitive, it shows the main idea of the sentence.
Lets see at this variant:

Принимал экзамены мой старый друг.

This means that text before already tell us about exams, reader know that there are(were) some exams. And, who did conduct it? My old friend! That is main idea of sentence.

- Кто принимал экзамены?
- Принимал экзамены мой старый друг.

Lets see another variant:
Мой старый друг принимал экзамены.
That means that text before tell us about old friend. And, what did he do? He conduct the exams, that what he did.

- Что вчера делал мой старый друг?
- Мой старый друг принимал экзамены.