Complex sentence: Crenation occurs because in a hypertonic environment (that is, the cell has less solutes than the extracellular fluid does) osmosis (the diffusion of water) causes a net movement of the water outside of the cell, causing the cytoplasm to decrease in size.
Split into simpler sentences: Crenation occurs because in a hypertonic environment (that is, the cell has less solutes than the extracellular fluid does). Osmosis (the diffusion of water) causes a net movement of the water out of the cell, causing the cytoplasm to decrease in size.