I have a really quick and easy way to remember this.
If a cell is placed in hypOtonic solution, it well swell up (water flows inside the cell. I remember this by the letter ‘O’ looking like a swollen cell. Also think hypo=low concentration of solutes meaning excess water outside the cell which will come in. Whenever you see hypo it means low. (Hypotension, hypotonic etc etc).
If a cell is placed in Hypertonic solution, it will crenate (shrink as the water flows out. You can also kinda look at the ‘e’ in hypertonic; the ‘e’ is kinda small and wrinkly like the cell when it shrinks down. Also the prefix ‘hyper-’ means something is high (hypertension=high blood pressure), which in this case means there is a high concentration of solutes that water will need to flow out of the cell to balance out.
This is a quick and easy way of working it out at a glance, but it’s always a good idea to know the reasons behind why it works because then you won’t need memory tricks. Understanding makes it easier.