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When speaking of flying a fixed wing aircraft, there are 4 main forces that effect your aircraft while in flight.

Thrust
Thrust is normally provided by a propeller mounted on an operating engine, but aircraft such as gliders use momentum and gravity to produce a forward motion. Thrust must overcome drag (see below) for the aircraft to move forwards.
Drag
There are various types and causes of drag. Most drag occurs from having a non-aerodynamic shape (a big flat windscreen usually isn't too aerodynamic), landing gear, flaps or other aircraft control surfaces, and the general drag caused by lift. Because there are so many different types and causes of drag, it is best you read the node devoted to it. If drag overcomes thrust, the aircraft will no longer move forwards, likely to cause a lack of lift as well.
Lift
Lift in most aircraft is generated by a low pressure area over a wing; this is described in more detail under the theory of flight node. Lift can also be caused by external factors, such as an upward-flowing wind or an upward air movement due to ground heating (thermals). Lift must overcome weight for an aircraft to become airborn.
Weight
The weight of the aircraft is the mass being acted on by gravity (see formulas and the like in the writeup "weight"). The weight of the aircraft affects the performance of the engine and thus thrust, as well as counteracting lift. Most smaller aircraft are just heavy enough to take off safely, so knowing how much your passengers weigh is very important.
This is the bread and butter of how and why things work in an aircraft, and it's best you understand these before you attempt any ground school education!

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