在Penn Treebank中提取包含介词“to”的所有句子

Question

我正在使用python做作业。我的想法是，我需要提取包含动词“给”和介词“到”的所有句子（例如，我将花束给予女孩）。我想出了一个代码，向我报告错误的缩进空间错误。我不知道出了什么问题。这是我写的代码：

import nltk

from nltk import treebank

for filename in nltk.corpus.treebank.fileids():

    files=open(filename,'r')

    file=files.read()

    for sentence in file:

        a=sentence.index('to')
        #a=the position in which 'to' appears

        b=sentence.index('give')
        #b=the position in which 'give' appears

        mylist=[]

        if a!=-1 and b!=-1:
                #if a and b exist simultaneously
                mylist.append(sentence)

                print(mylist)
                new_file_name='List'+'.txt'
                #create a new file

                file=open(new_file_name,'w')
                for sentence in file:
                    file.write(sentence)
                    file.write('\n')
                file.close()

有人可以帮助我吗？