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.
His is confusing, since both the pronoun and the adjective are the same word. Try substituting in hers, theirs, ours, yours, or mine if you aren't sure:
The black car next the bakery is his.
"The black car next to the bakery is her?" No. "The black car next to the bakery is hers." It's the possessive pronoun.
In general, the possessive adjective is used when directly modifying a noun, and the possessive pronoun is used when it is either the subject or the direct object of the sentence.
More examples:
Adjective: His life is full of adventure. (Her life is full of adventure.)
Pronoun: His is a life full of adventure. (Theirs is a life full of adventure.)
Pronoun: That thing? It's his. (That thing? It's mine.)
And so on.
Best Answer
The basic "rule" is that formal written English prefers the possessive in all circumstances, while informal, conversational speech prefers the objective. Generally, the more formal the register, the more likely it is that the possessive will be used. However
Formal English is becoming less rigid. It now tolerates the objective in many circumstances—for instance, a transitive gerund clause which acts as the complement of a matrix verb, as in snailplane's example:
Likewise, the possessive is hardly rare in informal English; it is particularly frequent when the gerund clause is the first constituent in a matrix clause:
What seems to govern colloquial use is the syntax; a gerund which plays a particularly ‘un-verbish’ and ‘noun-y’ role is more likely to take the possessive.
So although some dogmatic ‘descriptivists’ claim that the possessive sounds ‘stilted’, in my experience this isn’t true; colloquial English tolerates a wide variety of usages.
Under the circumstances, I would advise you to stick to the possessive, particularly if you are in school or are in a field which demands a lot of formal writing. It’s easier to make a habit of using just one form instead of trying to juggle two; and the possessive will never get you into trouble.