Sure, here are the completed sentences with comparative and modifying words:

1. Mr. Brown is 50 years old. Mrs. Brown is 49 years old. Tom is 10 years old.
- Mr. Brown is older than Mrs. Brown.
- Mr. Brown is much older than Tom.
- Tom is younger than Mr. Brown.
- Mrs. Brown is older than Tom.
- Tom is much younger than Mrs. Brown.

2. This street is 2kms long. That road is 40kms long.
- This street is shorter than that road.
- That road is much longer than this street.

3. Today the temperature is 23 Celsius degrees. Yesterday the temperature was 24 Celsius degrees.
- Today it is cooler than yesterday.
- Yesterday it was warmer than today.

4. The temperature in Sapa in winter is 2-3 Celsius degrees. The temperature in Ninh Thuan in winter is 20-25 Celsius degrees.
- Winter in Sapa is colder than in Ninh Thuan.
- Winter in Ninh Thuan is warmer than in Sapa.

5. The dress is $580. The pair of shoes is $210. The hat is $500.
- The dress is more expensive than the hat.
- The hat is cheaper than the dress.
- The pair of shoes is less expensive than the hat.
- The hat is more expensive than the pair of shoes.
- The pair of shoes is much cheaper than the dress.

6. The green box is 2kg. The red one is 15kg. The black one is 16kg.
- The green box is lighter than the red one.
- The red box is heavier than the green one.
- The black box is heavier than the red one.
- The red box is lighter than the black one.
- The black box is much heavier than the green one.