=>

a. We can't see the stars at night due to the light pollution.

b. The last time Phong visited Tasmainia was 15 years ago. / Phong last visited Tasmania 15 years ago.

c. By the time he came to my house last night, I had left.

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.