Someday means at some time in the future, but you do not know when — and usually you are thinking of a time quite far away. It carries feeling in a way that 'in the future' does not. When you say 'someday I'll visit Iceland', you are talking about a hope: something you want, that has no date, that may or may not happen. That is why the word is all over songs and speeches — 'someday we'll be together', 'someday things will be better'. It can also carry a warning, said to a child or a friend: 'someday you'll understand why I said no'. Two things about how it works. First, it goes with the future, so you need will or going to, not the present: 'someday I will learn to drive'. Second, keep it separate from 'some day' written as two words, which means one particular day that hasn't been chosen yet — 'let's meet some day next week' means one of those seven days, and nobody is dreaming about anything. The one-word spelling started as American and is now normal everywhere, though British writers still often prefer 'one day' for the same meaning. If you want to sound less certain still, 'someday, maybe' is very common. And if you want to sound doubtful about someone else's plan, English has a dry little phrase: 'someday never comes'.