MatterMatter can be a problem or difficulty. As a verb, matter means to be of importance.
Ex: The matter with puppies is that they're difficult to housebreak, so the owner must be careful to take them outside frequently to avoid the puppies' making messes in the house.
Ex: My children's happiness is more important than anything else in my life. It really matters, but I do not want to spoil them. I find myself telling them that, when I refuse them something, it's for their own good.
As a matter of fact means actually or in fact. Additionally, a matter of course means routinely or normally, as a normal procedure.
Ex: Sylvia cannot understand how she failed her driver's test. As a matter of fact, she believes that the examiner failed to notice that she used her signal light when making the left turn.
Ex: Every week, as a matter of course, I put ten per cent of my salary in a savings account so that, one day, I can replace my ten-year-old car.
ConcernConcern is worry, so if you are concerned about someone or a situation, you are worried. As a noun, concern is the situation about which one is worried.
Ex: Brian is concerned that his spotty employment record will make getting a new job difficult. Most employers look for workers who have stayed at one job for a long time.
Ex: Brian's concern is realistic based on his father's experience in the world of work. His father has never stayed in one place very long and has difficulty finding work.
To whom it may concern is a common form of address when a letter writer does not know the identity of the person or persons to whom she or he is writing. Concern is also another term for a business or other endeavor.
Ex: Before women made it to the top of the corporate ladder, most people addressed letters with "Dear Sir", but now it is more appropriate to use "To Whom it May Concern" which applies to both men and women.
Ex: Sam Walton opened the first Wal-Mart in the early 1960s, and since then the discount department store has become an international concern with stores all around the world.