numeral
first
A1
Erste is the ordinal 'first' (numeric value 1). Used attributively (der erste Tag) or predicatively (Er war der Erste). It inflects for gender, case and number (erste, erster, erstes, etc.). Common in dates (am ersten) and rankings. Not irregular; endings follow regular adjective/numeral declension patterns. Remember agreements with nouns always.
Examples
Der erste Versuch gelang, obwohl die Bedingungen schwierig waren.
The first attempt succeeded even though the conditions were difficult.
Das ist die erste Seite des Buches.
That's the first page of the book.
Details
Mnemonics
imagine a ribbon with '1.' and the word 'erste' on it.
think 'first' → 'erst(e)' — 'erste' looks like 'first' with an e.