How to determine the complexity of an English sentence?
Yes. There are various metrics, such as the fogg index.
Textacy in python has a nice list and implementations.

&gt;&gt;&gt; ts.flesch_kincaid_grade_level
10.853709110179697
&gt;&gt;&gt; ts.readability_stats
{'automated_readability_index': 12.801546064781363,
 'coleman_liau_index': 9.905629258346586,
 'flesch_kincaid_grade_level': 10.853709110179697,
 'flesch_readability_ease': 62.51222198133965,
 'gulpease_index': 55.10492845786963,
 'gunning_fog_index': 13.69506833036245,
 'lix': 45.76390294037353,
 'smog_index': 11.683781121521076,
 'wiener_sachtextformel': 5.401029023140788}