1. It has been 3 years since my brother went to Oxford.
→ My brother went to Oxford 3 years ago.
2. My dad last smoked 4 years ago.
→ It has been 4 years since my dad last smoked.
3. Giang hasn’t heard this song since he was a boy.
→ Since he was a boy, Giang hasn't heard this song.

1. It has been 3 years since my brother went to Oxford.
 
→My brother went to Oxford 3 years ago.
 
2. My dad last smoked 4 years ago.
 
→ It has been 4 years since my dad last smoked.
 
3. Giang hasn’t heard this song since he was a boy.
 
→ Since he was a boy, Giang hasn't heard this song.

 

1. It has been 3 years since my brother went to Oxford.
 
→  My brother has gone to Oxford for 3 years / The last time my brother went to Oxford was 3 years ago. / My brother last went to Oxford 3 years ago. 
 
2. My dad last smoked 4 years ago.
 
→ it's 4 years since my dad last smoked.
 
3. Giang hasn’t heard this song since he was a boy.
 
→  Giang last heard this song when he was a boy.