How do you use the possessive form?

The simple answer is this: In English, we indicate the possessive (what belongs to someone or something) by putting an apostrophe followed by an "s". So a shoe belonging to Susan would be "Susan's shoe"; or the head of a doll would the "doll's head."

However, in the case of a plural noun or a noun ending in "s" the "s" is replaced by an apostrophe only. For example the trucks belonging to more than one boy would the "the boys' trucks"; a dog belonging to Titus would be "Titus' dog."

In the case of possessive pronouns, an apostrophe is not used because they already indicate possession; hence his, hers, its, yours, ours, theirs, and whose are all correct; "it's" is always a contraction of "it is."