My question is about the ownership s. It's, is there any clear rule about when we should add the ownership s to words please?
Do you mean the possessive form of words? The general rule is to add
's to the end of a word. A few notes:
However, only an apostrophe is added to words that end with s: James' book, the girls' class.
Just to explicitly state, the criterion here is the ending s, not whether or not a word is in plural.
'smust be added to plurals that do not end with s: e.g. the children's choir.
'sas well for words which are pronounced with a /s/ (or /z/) sound at the end but whose spelling don't end in s: such as the mice's hideout. In such cases, it might be better to avoid the possessive form, compare Illinois' surface vs. the surface of Illinois; the first option sounds a little awkward.
The possessive form of pronouns do not have an apostrophe: yours, his, hers, its, ours, theirs. Do not the form of its vs. it's: It's interesting how the bird feeds its offspring.
If you're unsure whether to use the possessive form at all, see this short answer (Student's Book vs. Student Book).
Something that may surprise English language learners is that if you are going to somebody's house, you have to use the possessive form as well, e.g. I'm going to my friend's, she's going to Bobby's.
Another heads up, note that some plurals do have an apostrophe, particularly acronyms and letters (for instance: this text only has two K's in it).