Questions & Answers about O cantil está cheio?
O means the, while um means a/an.
So o cantil refers to a specific canteen/flask that both speaker and listener already know about.
- O cantil está cheio? = Is the canteen full?
- Um cantil está cheio? would sound more like Is a canteen full?, which is much less natural in most situations.
Cantil usually means a canteen or flask, especially the kind used for hiking, camping, military use, or carrying water.
In everyday Brazilian Portuguese, people might also use other words depending on the object:
- garrafa = bottle
- garrafinha = small bottle
- squeeze = sports water bottle
So cantil is correct, but it refers to a fairly specific type of container.