1)I/just/my classmate.(seen) 2)My parents/just/a big meal.(have) 3)My sister/a new mobile phone .(buy) 4)My best friend/just/some

1) I/just/my classmate.(seen) — I have just seen my classmate.

 2) My parents/just/a big meal.(have) — My parents have just had a big meal.

3) My sister/a new mobile phone.(buy) — My sister has bought a new mobile phone.

4) My best friend/just/some good news.(hear) — My best friend has just heard some good news.

5) I /my exam.(pass) — I have passed my exam.