possessives - When do we use apostrophe s?

I always feel uncertained when using apostrophe s. For example,

company's rules.

and

company rules.

I am not sure when to use apostrophe s. More examples

The Pepsi's logo.

vs

The Pepsi logo.

or

The company's sales raise ...

vs

The company sales raise ...

Or

Thai culture.

vs

Thai's culture

or even

Thais' culture.

Answer

In general, you can use both forms with pretty much the same meaning. The way you would word the phrase depends on whether you're using the possessive (apostrophe s) or not. For example, you would say

the company's rules

or

(the) company rules

In the first case, you're talking about the rules of the company by using the possessive form of "the company". In the second case, "company" is an attributive noun, which means that it modifies "rules" sort of like an adjective. "Company rules" is a noun phrase that means the rules of the company. You can use "the" in that case but it isn't strictly necessary. For example:

Company rules state that every employee must be given 30 minutes for lunch.

With the Pepsi logo example, you would say

Pepsi's logo

or

the Pepsi logo

You wouldn't put "the" in the first case because you don't put "the" before the name of a company any more than you would put "the" before a person's name. (Of course, some companies have "The" at the beginning of their names, in which case you would use it.) In the second case, "Pepsi" is being used attributively to modify "logo". "The" (or another article) is needed in that case. (I don't have an explanation for why you need an article there but not in the "company rules" example above.)

For the "Thai culture" example, you would use that or "the Thai people's culture", or possibly "the Thais' culture" (although that last option sounds awkward to me).