What is inertia?

For ordinary terrestrial experiments,the observer in an inertial frame in the following cases is

When a car at high speed makes a sharp turn,the driver in the car tends to get thrown in the direction opposite to that of the turn. This is due to which property?

$Assertion$ : The driver in a vehicle moving with a constant speed on a straight road is an inertial frame of reference.
$Reason$ : $A$ reference frame in which Newton's laws of motion are applicable is non-inertial.

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R.$
Assertion $A:$ An electric fan continues to rotate for some time after the current is switched off.
Reason $R:$ The fan continues to rotate due to inertia of motion.
In the light of the above statements,choose the most appropriate answer from the options given below.

Write and explain Newton's first law of motion.