Translation:The girl gives the boy the apple because she is kind.
Would it not be more correct to say that we use "fordi" because the INTENT is to create a main clause and a subordinate clause? At least in English it is perfectly acceptable to say "The girl gives the boy the apple for she is kind". Using "for" simply changes the emphasis.
Furthermore, there is an actual linguistic reason behind this; it's not just random. The softer "g" sound is further towards the front of the mouth (palatal), not in the back (velar). This is due to a phonological process called assimilation in the study of linguistics. When followed by a front vowel (one pronounced towards the front of the mouth), such as "i", "y", or (in Norwegian) "j", the preceding consonant (in this case "g") moves towards the front of the mouth as well to match the vowel's position, and it becomes more palatalized. When followed by a back or mid vowel such as a, e, or u, the "g" retains its original velar (towards the back of the mouth) quality.
Sorry for using a few technical linguistic terms here. But I hope that understanding WHY the "g" sound changes in front of certain vowels will help you to remember which ones they are, instead of just having to memorize them.
Well, the second phrase is dependent on the first phrase, it wouldn't exist on itself... Imagine 2 phrases, The girl is kind. The girl gives the apple to the boy. Sounds broken, because the first phrase is out of place, so it is subordinated to "The girl gives the apple to the boy"