Complex sentence: A photon that has been'marked' cannot interfere with itself and will not produce fringe patterns, but a photon that has been'marked' and then'unmarked' can interfere with and will produce the fringes characteristic of the Young experient.
Split into simpler sentences: A photon that has been'marked' cannot interfere with itself, but a photon that has been'marked' and then'unmarked' can interfere with itself. If a stream of photons has been'marked' and then'unmarked' it will produce the fringes characteristic of the Young experient.