This applet should be useful in picking up vector addition and subtraction using triangle or parallelogram laws.
To add using the triangle law the vectors have to be arranged in order, that is with tail of one touching the tip of another. The sum vector is drawn if the you arrange the vectors so.
To subtract using triangle law the vectors have to be arranged in improper order, that is, tips or tails of the two vectors should be made to touch. Again the difference vector is drawn when the vectors are properly arranged.
Let me explain the subtraction part a bit. When you have A and B in improper order the vector joining the free ends is A - B or B - A depending on the direction. This vector is A - B, if it is in order with B and B - A, if it is in order with A. In the applet the vector A - B is being drawn. So this would be in order with B.
While using parallelogram law only add option is available. The vectors in this case must be arranged with tails touching. The parallelogram is drawn and the resultant is shown.
So draw away merrily and get a feel for the addition and subtraction of vectors.
To add - using triangle law the vectors should be in order (tail of one touching the tip of another) - using parallelogram law the tails must touch each other.
To subtract - the vectors should be in improper order ( tails of the two vectors or tips of the two vectors should be touching each other) - This option is not available with parallelogram law.