Generally it's good to say inshallah ("Allah willing") when saying one will do things in the future, for example: "inshallah, I'll go to Egypt one day"; "inshallah, I'll graduate from college"; "I'll go to the dentist after work, inshallah"; "inshallah, I'll find a nice husband".
And never say of anything, "Indeed, I will do that tomorrow," Except [when adding], "If Allah wills." And remember your Lord when you forget [it] and say, "Perhaps my Lord will guide me to what is nearer than this to right conduct." -- Qur'an 18:23-24
Sometimes we talk about acts we are intending not to do, for example: "I'm not going to that conference in New Zealand"; "I'm not paying for that"; "I'm already learning Spanish, I'm not going to learn Korean too"; "I'm never getting married". It's seemingly out of place to say 'inshallah' in these instances, despite it being possible for Allah to intervene if He willed.
Question: Should one say inshallah for acts they're intending not to perform?
There seems to be a curious distinction between the two, which I'm finding tricky to separate.