"I am going to miss you."
Translation:Jag kommer att sakna dig.
'Ska' usually carries the meaning of someone wanting something to happen, or controlling what's happening. In Swedish, we don't think we can control whether we're going to miss someone or not. It's not like I decide to miss you, it's just going to happen, therefore it should be kommer att.
- fattas: there's supposed to be something here, but it's missing
- saknar: as in "I miss him" or "I am missing five chairs"; note that saknas can be used like fattas
- missar: as in "the football player misses the goal"
- tappar: means to drop or to lose something
There are some additional uncommon meanings I didn't include as well.
Yes but you know wether or not you will miss someone so ska seems the logical choice here, whereas something completely unpredictable like the weather det kommer att regna i morgon Kommer att would be used. You say that missing someone is out of your hands but it is inevitable, and would be if you were saying this phrase, no?