Whats wrong with the expression a[i]=i++; ? Whats a sequence point?

Whats wrong with the expression a[i]=i++; ? Whats a sequence point?
Although its surprising that an expression like i=i+1; is completely valid, something like a[i]=i++; is not. This is because all accesses to an
element must be to change the value of that variable. In the statement a[i]=i++; , the access to i is not for itself, but for a[i] and so its
invalid. On similar lines, i=i++; or i=++i; are invalid. If you want to increment the value of i, use i=i+1; or i+=1; or i++; or ++i; and not some
combination.
A sequence point is a state in time (just after the evaluation of a full expression, or at the ||, &&, ?:, or comma operators, or just before a
call to a function) at which there are no side effects.
The ANSI/ISO C Standard states that
Between the previous and next sequence point an object shall have its stored
value modified at most once by the evaluation of an expression. Furthermore,
the prior value shall be accessed only to determine the value to be stored.
At each sequence point, the side effects of all previous expressions will be completed. This is why you cannot rely on expressions such as
a[i] = i++;, because there is no sequence point specified for the assignment, increment or index operators, you don't know when the effect
of the increment on i occurs.
The sequence points laid down in the Standard are the following:

1.The point of calling a function, after evaluating its arguments.
2.The end of the first operand of the && operator.
3.The end of the first operand of the || operator.
4.The end of the first operand of the ?: conditional operator.
5.The end of the each operand of the comma operator.
6.Completing the evaluation of a full expression. They are the following:
1-Evaluating the initializer of an auto object.
2-The expression in an ?ordinary? statement?an expression followed by semicolon.
3-The controlling expressions in do, while, if, switch or for statements.
4-The other two expressions in a for statement.
5-The expression in a return statement.