Ein vs. eins
The German words ein and eins can both translate to one in English, but they have distinct uses in grammar and syntax, carrying different connotations depending on their placement and application in a sentence.
Ein
A1Ein is used as an indefinite article similar to a/an in English. It also translates to the number one when qualifying masculine and neuter nouns.
Ich habe ein Buch.
(I have a book.)
Ein Apfel ist auf dem Tisch.
(An apple is on the table.)
Er hat nur ein Euro.
(He only has one Euro.)
Eins
A1Eins is primarily used as a numeral, equivalent to the standalone number one in English. It's also used colloquially as a placeholder when the gender or nature of an entity isn't known or specified.
Eins plus eins gleich zwei.
(One plus one equals two.)
Kann ich auch eins haben?
(Can I have one too?)
Auf einer Skala von eins bis zehn, wie würdest du es bewerten?
(On a scale of one to ten, how would you rate it?)
Summary
Ein and eins serve different functions in German language despite both translating to one. Ein serves as an indefinite article for masculine and neuter nouns while eins primarily indicates the numeral one although it can be more flexible in its use.