Complex sentence: In order to understand Plato more clearly, it is useful to bear in mind his theory of Forms, according to which all the phenomena perceived by the senses are imitations of eternal and perfect Forms that alone have reality.
Split into simpler sentences: In order to understand more clearly Plato, it is useful to have in mind his theory of Forms. According to it, all the phenomena perceived by the senses are imitations of eternal and perfect forms that alone have reality.