The Greek Alphabet
Modern Greek is written in a 24-letter alphabet (Α α … Ω ω). Several letters look familiar from maths and science, but many sound different from their Latin lookalikes. One spelling quirk to learn first: the letter sigma is written ς at the end of a word and σ everywhere else (e.g. δάσκαλος ends in ς).
The rule
Greek uses a 24-letter alphabet (Α α … Ω ω). For lookup, the trap to remember is sigma: it is written ς word-finally and σ everywhere else - the same letter, two shapes. So δάσκαλος ends in ς, but the σ inside δάσκαλος is the ordinary form. This is purely orthographic: ς and σ are never a meaning difference.