Best corrected sentence: While the city sleeps, these machines sweep, pick up trash, and fix cracks, so the city looks nice by morning.
Another good option (two sentences): While the city sleeps, these machines sweep, pick up trash, and fix cracks. The city looks nice by morning.
Step-by-step explanation:
- Introductory phrase: "While the city sleeps" is an introductory clause. Put a comma after it: While the city sleeps,
- Subject and verbs: The subject is "these machines." The verbs are "sweep, pick up, and fix." Because the subject is plural (machines), the verbs stay in the simple plural form — that is correct.
- List commas: When you list actions, put commas between them. The comma before "and" (called the Oxford comma) is optional but helps make the list clearer: "sweep, pick up trash, and fix cracks."
- Using "so": "So" connects the two ideas (machines work at night → city looks nice). You can join the ideas with a comma before "so" or make them two sentences. Both are correct; joining shows they happen because of each other.
Quick note: Starting a sentence with "So" is okay in casual writing, but joining the sentences with a comma is a little more polished.