Normal force != mg on an incline, in an elevator, or when additional vertical forces act. N = mg only on a horizontal surface with no vertical applied force.

Action-reaction pairs act on DIFFERENT bodies. Weight and normal force on a book are NOT action-reaction (both act on the book).

Static friction direction can be up OR down an incline depending on the applied force. It opposes the TENDENCY of motion, not necessarily the applied force direction.

Massless string tension is the same throughout only if the pulley is massless and frictionless. With a massive pulley, tensions differ on the two sides.

"ma" is not a force — it is the NET effect of all forces. Never include "ma" in the FBD.

When blocks move together: use system approach for acceleration, individual FBD for internal forces.

Friction can cause motion — e.g., friction on a box in an accelerating truck causes the box to accelerate forward.

Maximum value problems: At the verge of an event (sliding, tipping, breaking), the limiting condition is an equality ($f_s$ = $mu_s$*N, T = $T_{max}$, etc.).