Que onda paisano! Ojalá estés bien. He estado aprendiendo español por unos años, no soy nativo pero creo que sí te puedo ayudar aprender si quisieras! What's up my fellow Americano! I hope you're well. I've been learning Spanish for a few years now, I'm not a native but I do believe I can help you learn if you would like!

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Firstly, I would say, the 3rd example is incorrect altogether. The difference between “the” and “a” is similar to は and が, in that, there’s a particular feeling of emphasis that each word imparts to a sentence. The air is sweet with the smell of spring. This sentence, to me, feels more like your emphasis is on the end of the sentence. You want the listener to imagine the smell of spring. The air is sweet with a smell of spring. This sentence feels more like your emphasis is on the beginning of the sentence. You aren’t trying to make the listener imagine the smell of spring. You want them to know the air smells sweet, like if it were springtime.

I agree with Profe Jose. There’s really no difference in the way those sentences would be understood. There’s a potential that: Are we going to get off at the next stop? Might sound like you’re uncertain about how to get the final destination. It could imply something like “I’ve never been to this place before, is this the correct stop? Is this when we’re supposed to get off the train?” But, that interpretation would be more based off your tone than your words.