Imagine the reaction a + b ⇌ c + d is at equilibrium, where the forward and reverse reactions are at equal rates. what would happen to the rate of the forward reaction if the [a] were suddenly to increase?

Rate of forward reaction will increase.

Explanation:

Effect of change in reaction condition on equilibrium is explained by Le Chatelier's principle. According to this principle,

If an equilibrium condition of a dynamic reversible reaction is disturbed by changing concentration, temperature, pressure, volume, etc,  then reaction will move will in a direction which counteract the change.

In the given reaction,

A + B ⇌ C + D

If concentration of A is increase, then reaction will move in a direction which decreases the concentration of A to reestablish the equilibrium.

As concentration A decreases in forward direction, therefore, rate of forward reaction will increase.