When the fulcrum is under the middle of the lever, how can the lever be kept balanced?
The lever will remain balanced, so long as the power and the weight used are equal to each other.
When the lever is moved, so that the fulcrum is nearer the weight, which arm of the lever is the heavier?
The arm towards the power, because it is the longer arm.
Will the power applied to the long arm of the lever be as heavy as the weight?
No; the nearer the fulcrum is to the weight, the less will be the power needed to balance that weight.
How will it be when the fulcrum is nearer to the power than it is to the weight?
Then the power must be heavier than the weight.
When the long arm of the lever is twice the length of the short arm, what will be the difference in the weights?
One pound applied to the long arm will then balance two pounds applied to the short arm.