After the incident, which caused his condition, he cannot remember anything more than a few minutes old, other than the events before that incident. Yet, he remembers his condition?
Leonard can not remember about his condition anytime. Even sometimes he forgets what he is doing right now. Remember the scenes when he is running because he is being chased or when he drinks the spitted beer. He just suddenly looses any clue why is he doing something in a time. After the accident, he said he can not store recent memories. But he can remember everything before the accident. But he has to be reminded of the incident (He does it by viewing his tattoos, notebook etc). In his tattoos he reminds himself everything. About the condition he reminds himself. Look carefully:
He remembered about Sammy Jenkins that who had a special condition.
He remembers about the accidents.
By remembering Sammy Jenkins, he remembers he has the similar condition.
He is now confirm about his weakness.
He reminds himself how he can try to remember things.
This is the way he reminds himself about his condition and the other facts he is looking for. Sam Jenkins is a fictional character through which he remembers he has the condition. Sammy is a character from Leonards memory where memory of Leonard and really Sammy has mixed.
All the tattoos listed by Mistu4u help Leonard in guiding himself to cope with his conditions.
However even in this condition he never forgets daily chores like driving, eating, shaving etc. He is also able to use English language comfortably. Through his subconscious he knows about his condition too.