What is the exact statement of Hooke's law? The statement of Hooke's law states that STRESS is directly proportional to STRAIN. But if stress depends upon strain then strain must occur before stress comes into play. I am unable to understand how actually it happens when anything is applied a force upon.  
 A: We can express Hooke's law mathematically as follows:
$$stress \propto strain$$
$$\frac{stress}{strain} = const$$
This constant is known as modulus of elasticity.

Stress is defined as force per unit area and strain is defined as change over actual value. Stress can cause strain (thermal stress) and strain can cause stress (stretching a string). 
We will examine one case. When you pull a string,  you are applying a force, i.e: you are stressing the material. The string responds by stretching; this stretching is known as strain.
Mathematically, the stress and strain in this case is given by:
$$stress = \frac{F}{A}$$
$$strain = \frac{\Delta l}{l}$$
According to Hooke's law, the ratio of the two aforementioned quantities us constant. In this case(longitudinal stress), the constant is known as Young's Modulus.
A: Whenever the spring is displaced from equilibrium by displacement $d$, there is a restoring force $-F_R$ towards the resting position. Hooke's Law only describes this restoring force $-F_R$. It doesn't describe external forces $F_E$.
Imagine you paused time (so the spring can't be displaced). When an external force $F_E$ is applied to the spring, you tend to displace the spring from equilibrium (but you haven't actually displaced the spring yet). This causes the spring to generate the restoring force $-F_R$ in the opposite direction of the intended displacement.
Now see what happens if you unpause time. An external force $F_E$ is applied to a spring at equilibrium. If $|F_E| > |F_R|$, the spring changes length with displacement $d$ proportional to $F_R$. If $|F_E| <= |F_R|$, the spring doesn't move.
Similar force analyses can be done for other starting positions of the spring (relative to the equilibrium position).
