For this discussion I choose a few sentences which I consider share important information.
The word FOR can be either a preposition or a conjunction:
The preposition for introduces a complement (noun/ pronoun / -ing form) indicating time and purpose.
"I have lived here for five years" = indicates how long an action lasts.
"The knife is for cutting bread" = explains what something is used for
The conjunction for links two clauses. It is very formal and usually introduces an additional reason, not the main cause.
"I believed her, for she had never lied to me before"
"He was tired, for he had worked all day."