I'll give you a quick 2nd answer to your follow-up comment.
The steps you want to take are:
1. move for loop into a function that takes min and max loop values
2. return the sum
3. call the function in an outer loop
4. each time the outer loop runs, print the sum value so you can see process running (with output) even though it will take a long time for the entire program to run.
I'm no Python wiz and this is untested code, but it'll look something like the following:
Loop will run 1-10, 11-20, 21-30, etc. to max (1000)
loopMultiplier = 0;
localSum = 0;
// this is the outer loop that calls the function doFib() multiple times
// each time thru the outer loop you will get a print so you can see it running
for x in range (1, 1000)
localSum += doFib((loopMultiplier *x) +1, loopMultiplier * x)
loopMultiplier += 10
print("the even terms sum is ",localSum)
def doFib(minVal, maxVal) :
n1 = 1
n2 = 2
sum = 0
for i in range(minVal,maxVal):
nth = n1 + n2
n1 = n2
n2 = nth
if n1%2 == 0:
sum += n1
elif n2%2 == 0:
sum += n2
return sum