numeral
forty
A1
'vierzig' is the cardinal number meaning 'forty'. It's an invariable numeral for counting and quantifying (vierzig Jahre). Not declined before nouns; can be nominalized or form ordinals (vierzigste). No plural form; used in compounds (vierzigjährig) and in expressions like 'in his forties' (in den Vierzigern).
Examples
Der Bus transportierte vierzig Personen, weil der andere Bus ausfiel.
The bus transported forty people because the other bus broke down.
Das Buch kostet vierzig Euro.
The book costs forty euros.
Details
Mnemonics
Picture the number 40 on a scale to remember 'vierzig'.
Sounds like 'vier-zig' — remember the 'zig' ending for tens like in English 'forty'.