Complex sentence: The lower wing was nothing more than an attachment for the wing struts; it was (almost) removed in the second series (I-4bis), used as a parasite fighter in Zveno experiments with TB-3 bomber.
Split into simpler sentences: The lower wing was nothing more than an attachment for the wing struts; it was (almost) removed in the second series (I-4bis). The aircraft was used as a parasite fighter in experiments with the TB-3 bomber.