What I mean is, for example the MD 80 aircraft have a wing placed way back near the engine why not put it on the center? but the Boeing 737 have a centered wing. I'm asking this because I love to make cardboard planes but I don't know where the best spot to place the wing, I've put the wing on the front the middle and at the back but I still don't know whats the different. anyone got answer.
The position of the wing is determined by many things, but primary among them is an analysis of the distribution of the weight of the aircraft. Aircraft remain stable in flight because there is a balance of moments (that is, the nose up/nose down rotational effects of the forces generated by the airflow over every part of the aircraft). The total moment generated by all the aerodynamic forces on the surface of the aircraft will cause the nose to pitch up, or pitch down, depending on where the total force vector from all these forces lies in relation to the center of mass of the aircraft (often referred to as Center of Gravity or CG). All moments rotate the aircraft around the CG. It's just like a balance beam, or a scale with sliding weights, except that instead of having a fixed point that everything rotates around, the rotation point is the center of Mass. If you move the sliding weight on a scale to the right, the beam rotates down on the right, until an equilibrium is reached. The same thing happens with aircraft. If everyone in an aircraft moves to the front, or if cargo shifts forward on a transport aircraft, the aircraft will pitch nose down due to the change in the position of the CG. The aircraft's pitch attitude is only stable when the total aerodynamic force vector passes directly through the CG.
(Bear in mind that this is a complex dynamic thing, if the vector is not aligned with the CG, and causes a pitch change, the change itself will also cause a change in the position and strength of the aerodynamic forces. For the aircraft to be stable, these changes should bring the force more closely into alignment with the CG, rather than further away)
So, to answer your question, the wings are near the middle on a 737, and nearer to the tail on a MD-80/DC-9, because that's where the CG is. With the exception of the engines, and fuel, the weight of an airliner is pretty evenly distributed from front to back. The engines and fuel comprise a significant percentage of the total weight. So The CG on a 737 is near the middle because that's where the engines are (slightly forward of leading edge of the wings). And the MD-80/DC-9s engines are mounted at the back on the sides of the tail section.
On your paper airplane, first determine where the CG is. Balance the aircraft of the edge of a ruler, or hang it from a thread. Then place the wing so that the point one quarter of the way from the leading edge to the trailing edge is slightly behind the CG. Adjust forward or backwards from there until it flies the way you want. (Alternatively, you can add or remove paper (weight) from the nose or tail of the airplane to create the same effect.)
Without entering into details of aerodynamics:
The weight of the aircraft must be balanced by lift so altitude can be maintained constant. Lift can be increased or decreased a bit when it's necessary to climb or descend.
For many problems, weight can be seen as acting at the center of gravity, even if weight isn't actually a single force acting at a single point.
For the same reasons lift can be seen as a force acting at the aerodynamic center (AC, also named center of lift), even if the aerodynamic forces occur at any point of the wing and other lifting surfaces.
As CG and AC are not coincident, a pitching moment develops, tending to move the nose either up or down, depending of the relative locations of the two points.
To cancel this undesirable moment, an opposite moment must be created by generating lift with the tail, either positive or negative.
The size of the horizontal empennage is selected to produce a given quantity of lift. The distance from CG determines the pitching moment.
The tail also creates drag, depending on its size and the amount of lift generated.
Aircraft designers take all these elements to create a safe stable structure, easily flyable, including with engines shutdown. The total drag must be limited as much as possible to use as less fuel as possible. This leads to different choices and different relative positions for CG and AC.
For the MD-80, engines are closer to the tail, moving CG location more aft than when the engines are under the wings. The AC-CG distance is increased, so the pitching moment created by the wing is also increased. To compensate, the wing is moved backward.
The horizontal surfaces of the tail are in a T-configuration, which is weaker than a regular low configuration. It's desirable to limit the force generated by the tail. Such constraint and many other similar ones play a role in determining the exact location of AC relatively to CG.