a) We can't see the stars at night. There is too much light pollution ( due to )
→ We can't see the stars at night due to the light pollution
b) Phong hasn't visited Tasmania for 15 years ( last )
→ The last time Phong visited Tasmania was five years ago.
c) When he came to my house last night , I had left ( by the time )
→ By the time he came to my house last night, I had left.