Printing every 10th result in an alternating ± series
The first thing you can do to make the sign change obvious is to use operand = -operand, this way you avoid the need for the multiplication operator. Also changing the operand name to sign can help.

You also don't need counter as it will be equal to i before the test. Just change the test to use the modulo (%) operator instead of reseting counter.

And using augmented assignment for result can lead to a more readable line too:

result = 0
sign = 1
for i in range(1, 101):
    result += sign * i
    sign = -sign
    if i % 10 == 0:
        print(result)


Now, this probably need to be made more reusable, this means building a function out of this code and avoiding mixing computation and I/Os:

def alternating_sum(start=1, stop=100, step=10):
    result = 0
    sign = 1
    for i in range(start, stop+1):
        result += sign * i
        sign = -sign
        if i % step == 0:
            yield result


Usage being:

for result in alternating_sum(1, 100, 10):
    print(result)




Also, depending on the problem at hand, you can leverage the fact that every two numbers add-up to -1. So every 10 numbers add-up to -5. If you don't need much genericity, you can simplify the code down to:

def alternating_every_ten(end=100):
    reports = end // 10
    for i in range(1, reports + 1):
        print('After', 10 * i, 'sums, result is', -5 * i)