Electric Fields

The electric field at a point is set by the source charges around it, $\vec{E} = \vec{F}/q$, so the test charge divides out and the field belongs to the location. The force on a charge placed there is $\vec{F} = q\vec{E}$ with the sign kept, so a positive charge is pushed along the field and a negative charge opposite to it. Field lines give the direction of that push, their density carries the strength, and contributions from several sources add as vectors. In a conductor at equilibrium the excess charge sits on the surface and the interior field is zero; a charged insulator keeps charge throughout its volume and has a field inside.

Six errors dominate. Reading $E = F/q$ as though a larger probe produced a larger field. Copying the field arrow onto a negative charge instead of reversing it. Treating a field line as the path a moving charge will follow. Reporting zero field at a point that merely falls between two drawn lines. Adding or cancelling field magnitudes without drawing the two arrows, which swaps the like-charge and unlike-charge midpoints. And carrying the conductor's zero interior field over to a charged insulator, where the charge stays where it was put.