Ответ:

  1. My sister speaks English worse than I do.
  2. A bus is faster than a tram.
  3. The tallest trees in the world grow in California.
  4. Who is the most attentive student in your group?
  5. It is autumn. Every day the air becomes colder, the leaves yellower.

Ответ: worse; faster; tallest; most attentive; colder, yellower.