Hi! Let's break this down.
For #4, we have the sentence, 'Don't just walk, run to the store before the sale ends.' The question asks if the comma belongs after 'walk', if at all. If you say the sentence aloud, after 'walk' is where you would pause. So, the correct answer is 'yes', because one would pause after 'walk'.
The same goes for numbers 5 and 6. Where would you pause? One wouldn't say, 'Fix the problem first explain, the problem later'. That just doesn't make sense.
It could also help you to think of a compound sentence. Can everything before the comma be a sentence? Like, 'Don't just walk' or 'Fix the problem first'? (yes!) That can also be a clue.