Here are the comparative forms for the adjectives listed:

| No. | Adjective | Comparative Adjective | Superlative Adjective |
|-----|------------|----------------------|------------------------|
| 1 | narrow | narrower | narrowest |
| 2 | historic | more historic | most historic |
| 3 | polluted | more polluted | most polluted |
| 4 | short | shorter | shortest |
| 5 | good | better | best |
| 6 | boring | more boring | most boring |
| 7 | peaceful | more peaceful | most peaceful |
| 8 | crowded | more crowded | most crowded |
| 9 | bad | worse | worst |
| 10 | modern | more modern | most modern |
| 11 | fast | faster | fastest |
| 12 | cheap | cheaper | cheapest |
| 13 | convenient | more convenient | most convenient |
| 14 | noisy | noisier | noisiest |
| 15 | heavy | heavier | heaviest |

Feel free to ask if you need any further assistance!