 Hi, I'm Shayna from espressoenglish.net and I'm going to explain the expressions up in the air and nail down. If you say that plans are up in the air or things are up in the air, it means that they are not yet confirmed. For example, if you're deciding where to go to university but you're waiting for some more information, such as if you have been accepted into certain universities or if you have gotten any scholarships, then you could say, my college plans are still up in the air because they have not yet been finalized. Up in the air is different from on the fence, which means undecided. On the fence means that you need to make the decision. But up in the air means that the decision or confirmation depends on some external factors or external circumstances. To talk about confirming plans that were up in the air, you can use the expression nail down. For example, imagine you're organizing a conference with multiple speakers and many details. If you nail down the list of speakers and the schedule for the event, it means that you confirm or finalize these arrangements. Now you know how to use the English expressions up in the air and nail down when talking about plans and decisions.