Hi! 1. The first is correct use of hyphens, because it's connecting the years, five-to-ten. The second option leaves a hyphen unconnected to anything, which makes me think there is a break in the sentence where you don't mean for there to be. 2. This one is complicated because it's about slang, where the normal grammatical rules don't really apply. You're right that it's "have fun" correctly, but (especially in songs) people like to twist meanings. "Ain't we got fun" isn't correct at all, but people understand the meaning (basically what you said, "Don't we have fun". It's not common, however. I've never heard anyone say it in conversation. It's more of a playful phrase that isn't meant to be grammatically correct. Hopefully this makes sense!

Hi! I would say the first option makes more sense, because you're saying that he gave up his position by agreeing to run as a candidate, not by refusing anything.

Hi! I'm a little confused by your use of "rapid" as a verb, do you mean "quicken" (to make something go faster)? You should also change "investment" into a plural, "huge investments", and add "the" before "city". Other than that, the sentence is great.

Hi! Your sentence makes sense and is correct, but I'd suggest simplifying it with something like "We should be more careful in the future to not make the same mistakes."