noun
glass, drinking glass
A1
'Glas' can mean 'glass' (material) or a 'drinking glass' (a vessel). It's neuter (das Glas) with plural Gläser—note the umlaut plural. Genitive des Glases. As a material it is often uncountable; as a container use ein Glas Wasser. No special prepositions.
Examples
Ich habe ein Glas.
I have a glass.
Gib mir bitte ein Glas Wasser.
Please give me a glass of water.
Sie stellte das Glas auf den Tisch, bevor der Wind es umwarf.
She put the glass on the table before the wind knocked it over.
Details
Mnemonics
Picture a clear drinking glass with the word 'Glas' etched on it
Glass ~ Glas — same root as English
das Glas — imagine a simple neutral glass to recall 'das'
Notes
Plural: 'Gläser' (with umlaut).