- the "glasses" that people wear to help them see are always plural.
- "a glass" is something you drink out of. More than one of them is called "glasses".
- "glass" is the material that windows, mirrors, and glasses are made of. If you want to count individual sheets or broken pieces of it, you say "a piece of glass" or "pieces of glass".
Here are some simple examples to help you remember:
I can't find my glasses.
Do you want a glass of water?
Watch out! There's glass on the floor.