Hello Paco, Nice to see you again ![Smile [:)]](/emoticons/emotion-1.gif)
"O knew him for five years." means OVER five years, (from year 1 to year 5)
"I knew him five years ago." Means I knew him THEN, at that time, five years ago.
"I have known him for five years." means the relationship has continued to the present time. Present perfect progressive.
Anon:- Know = Present
Knew = Past
Knowing = Present Participle
Known = Past Participle, as in "Girls I have known."
Future tense:- "By the time we get there I know the show will have started". (something is going to happen in the future)