Changing the value of a for loop index on the fly

[Gabriel F. Alcober]

Hi! There goes a newbie trouble:

for i in range(0, len(subject)):
        if subject[i] in preps:
            psubject.append(noun_syn_parser(subject[0:i]))
            subject[0:i] = []

Since the last line eliminates some elements of the list, I'm wondering 
if it's somehow possible to change the value of "i" to 0 in order not to 
get an index error. Any other ideas?
Thanks in advance.