Here are the words or phrases that need correcting in each of the sentences:

1. **"what"** - should be **"to"** (correct phrase: "what to start").
2. **"play"** - should be **"playing"**.
3. **"is"** - should be **"the more dangerous"** (correct phrase: "the more dangerous it is").
4. **"doesn't"** - should be **"don't"** (correct phrase: "Sky trains don't get stuck").
5. **"is"** - should be omitted or rephrased (correct phrase: "The bigger the city, the noisier it becomes").

Each correction improves the grammatical structure of the sentences.