In the song "光るなら" there is a lyric that is:
"君だよ、君なんだよ"
When I translate it, it translates to: It was you, It was you!
Another example is in this lyric from the song "Centimeter" :
"運命なんて言えない"
Which translates to: I can't tell fate. But without the "なん" it seems like the same thing.
I think that it's changing the delivery of the sentence rather than the meaning, like ね or よ.
Thanks ☺️
なんだ and なんて are different things.
This なんだ is an informal variant of なのだ, which is explained in this answer: What is the meaning of ～んです/～のだ/etc?
Grammatically, it's made of three words, な + ん + だ, where な is the attributive form of the copula だ, ん is the noun called "explanatory-no", and だ is another copula. The literal translation of "君だよ、君なんだよ" is "It is you, it is that it is you".
As for this なんて, see: Usage of なんて and なんか as emphasis
This question is difficult even if I am Japanese...
In the first expression, なん is used to enphasize the phrase with strong feeling like anger, sadness, expectation and so on. If I translate, "It is you. It is really you！" However, we do not usually use this type of enphasis.
Speaking of the second one, difficult. We can say 運命とは言えない, 運命などと言えない instead, but why we choose one of these? I do not know, either. lol Anyway, I think we donot use this type of phrase as the first one. Only used in songs, literature and so on. Normally, we say 運命とは言えない or 運命とは思わない instead. translate: I cannot say it is fate.
If you are a beginning student of Japanese, I suggest that, for now, you think of these as two different "nans" and learn them separately.
The first nan is common and is used for emphasis. K-Yuuma's translation is very good. In other words, "It's you. It's really you." This nan is used commonly when there's an expectation that the people talking share the same information. So, another way to translate it might be "It's you. It's you [You know that's how I feel--I've demonstrated it or told you before]. Here's another example: think of a song where a women says she's going to leave a man. The man says "You can't leave me. I love you. You know I love you!" That is the function of nan in the your first example—to emphasize the speaker's emotions. Both of us know I love you! (or that you are the one for me!)
The second case is different. In this case, the nan still conveys emotion. But it carries a sense of disbelief or dismissiveness.
"I can't say it's fate" might be a way to translate it, but this fails to convey all of the nuance.
What they mean is "It might be fate, but I [just can't believe it]. Or [You might think it's fate, but I'm not so sure]. Or [People might call it fate, but who the heck are they to say? I don't really know the reason, but it ain't fate, that's for sure].
