Learn English – Is the word “student” an indefinite pronoun

The word your already denotes the possessive. In English, possessive adjectives do not need 's at the end, the adjective does that job along with identifying the owner.

I am my own friend.
I am your friend.
I am his/her/its friend.
I am our friend. (Doesn't make much sense, but it's grammatical and shows the pattern.)
I am your friend.

To keep things confusing for our English Language Learner friends, English does use a plain s (usually) at the end of the possessive pronouns to change them from being adjectives to nouns.

Whose friend are you?
I am mine.
I am yours.
I am his/hers/its.
I am ours. (And again, nonsensical, but grammatical.)
I am yours.

In short, your's or his's is never grammatical, nor would yours's or hers's be.

Regarding your second question, the common English pronouns are:

• Person: Subject, Object, Possessive Adjective (aka Personal Pronoun or Personal Determiner), Possessive Pronoun
• 1st person singular: I, me, my, mine
• 2nd person singular: you, you, your, yours
• 3rd person singular: he/she/it, him/her/it, his/her/its, his/hers/its
• 1st person plural: we, us, our, ours
• 2nd person plural: you, you, your, yours
• 3rd person plural: they, them, their, theirs

Note: This is in no way intended to be a full list of English pronouns; there are a fair many more. (Ex: This, that, these, someone, etc) However, the others are either treated as normal nouns when making possessive forms or they don't have sensible possessive forms at all. These are the ones that seem to be causing you confusion.

Without researching the words' etymologies, that's pretty much all I can tell you. English doesn't distinguish between singular and plural in the second person at all, and those are the pronouns we use. The Wikipedia article for this explains some of the history, but it doesn't include any real rationale for why the current forms are what they are.

The God of the Abrahamic religions (Christianity, Judaism, and Islam, basically) is traditionally a father figure, and the pronoun used is "he" (or "He", if you're using the majestic capital). Sometimes people will subvert this and use "She", either as a tongue-in-cheek substitution or as an earnest attempt at gender inclusivity, but in general, if you're talking about the God of a monotheistic religion, the pronoun to use is "He".

If you're talking about a higher being of a more personified pantheon (e.g. Greek or Norse myth), then use the pronoun appropriate to the gender of said higher being.

The one pronoun you never use for God, any God, is "it".¹

_{¹ Well, OK, maybe Cthulhu. Sometimes. But it's best to just not talk about him/her/it. :)}