Complex sentence: So even though there is only one photon in the experiment, because of its encounter with the half-silvered mirror, it acts like two and after being reflected off the ordinary mirrors, it will interfere with itself as if it were two different photons.
Split into simpler sentences: So even though there is only one photon in the experiment, because of its encounter with the half-silvered mirror, it acts like two. When "it" or "they" are reflected off the ordinary mirrors, it will interfere with itself as if it were two different photons.