His, her, your, their. (all apply here)
"His" _Su felicidad (de él)
"Her" _Su felicidad (de ella)
"Your"_Su felicidad (de usted, ustedes)
"Their"_Su felicidad (de ellos).
Note: "sus" (used with count nouns): eg. "Their things" are in the garage: "Sus cosas" están en la cochera. "felicidad" (none count noun). I hope it helps, it is difficult to give an explanation in English.
"Tú" : used with "you" singular informal, instead of "you" _"Usted" singular formal. eg,
You are a student: Tú eres estudiante.
You are teacher: Usted es maestro.
It depends if the person who you are talking to is your friend, you refer to him as ( Tú ). or someone like your boss ; we usually use "usted" .
I hope you understand. ( a little bit )? Greetings.
Because "a mi" is an indirect object, used with verbs that need indirect complement (ex: gustar).
Don't think of translating "to" with "a", it's the reason for your confusion. Prepositions can't be translated directly from a language to another one, it's better to think about the logics behind the use of the preposition in one language or in one other.
Here: Important for who? For me. For=para= indicator of the "addressee".
No. "-ness" can be "-dad", "-ción", "tud", etc... (but often, you have "-dad" for "-ty")
The reason, is the formation of substantive based on adjective in English is not regular. English has two main roots, Germanic adjectives, and French adjectives, and their formation in substantive often doesn't have the same rules.
Happiness (from happy, Germanic adjective)
Liberty (from "libre" and "liberté", French adjective and noun). All the words ended with "-ty" are form French words ended with "-té" in French and it's the reason why they are almost always "-dad" in Spanish, because Spanish and French have the same mother, Latin.
Freedom (from "free", Germanic adjective). None of the word ended with -dom are from French roots, they are all Germanic.