difference between よ and んだ

Question

What's the difference in using んだ and よ to end a sentence? For example:

嬉しいよ

嬉しいんだ

Both of them sorts of either answers a silent question or gives new/sometimes unexpected information about the sentence if I am not mistaken

Answer 1

嬉しいんだ is the casual (spoken) form of 嬉しいのです. This form is used for a variety of reasons but primarily to give a reason or explanation (or ask for one). It can be unnatural if used elsewhere so is best avoided if you are not confident with it (の also has uses as a nominaliser, possessive particle, and pronoun; these are all different). In this case 嬉しいんだ would be explaining that the reason for something (such as your behaviour) is happiness.

嬉しいよ is the casual form of 嬉しいですよ. Similarly 嬉しいね is the casual form of 嬉しいですね. These are used more often in casual situations than formal or written Japanese (except maybe for dialog). These are often translated to "you know?" (for よ) or "isn't it?" (for ね) but they aren't strictly questions. They also have many uses but are usually used when it is expected that the listener will agree with them. よ gives more emphasis and sounds "rough" so it's more often used by Japanese men. よ is used more for asserting things they assume they listener will agree with but may not be aware of. ね is expected the listener to respond by agreeing but can be used rhetorically like "right?" or "aye" in English. ね is also fairly colloquial and used to show empathy or invite the listener to respond or continue speaking. These are both quite common when chatting in Japanese but I think using ね is less likely to offend than よ. So 嬉しいよ could be used to say "this makes me really happy" and 嬉しいね could be used to say "that makes you happy, right?".