
eine
a / an
Indefinite article that accompanies a feminine noun in the singular nominative or accusative.
Ich sehe eine Katze auf der Straße.
I see a cat on the street.
a / one
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a / an
Indefinite article that accompanies a feminine noun in the singular nominative or accusative.
Ich sehe eine Katze auf der Straße.
I see a cat on the street.

one
Cardinal number for the digit 1 when it refers to a feminine noun and is stressed.
Es gibt nur eine richtige Antwort.
There is only one correct answer.

one
Indefinite pronoun that stands in for an unspecified feminine person or thing.
Eine von uns muss die Entscheidung treffen.
One of us has to make the decision.
The form 'eine' (a/one) is the pattern for the feminine form (nominative/accusative) of the 'ein' words, such as 'keine' (no/not any) and the possessive articles (meine, deine, seine). In these cases, they all end in '-e'.
Das ist eine Lampe. Das ist meine Lampe. Das ist keine Lampe.
That is a lamp. That is my lamp. That is not a lamp.
Before a feminine noun in the singular, the form 'eine' (a/an) is used in both the nominative and the accusative. You do not need to change the article in these two cases.
Eine Frau wartet an der Haltestelle.
A woman is waiting at the stop.
When 'eine' (one) is strongly stressed, it is the number word '1' and not the indefinite article. The context and the stress show the exact meaning.
Ich habe noch eine Frage.
I have one more question.
'Eine' (one) can also stand alone and then replaces a previously mentioned or known feminine noun. It then functions as a pronoun.
Brauchst du eine Tasche? – Ja, ich nehme eine.
Do you need a bag? – Yes, I'll take one.
someone
Here, eine and jemand both refer to an unspecified person. Eine assumes that a recognizable group of females is being referred to and often sounds more colloquial. Jemand does not specify a gender and also works without such a group.

Von den Kolleginnen hat vorhin eine angerufen.
One of the female colleagues called earlier.
Aus dem Büro hat vorhin jemand angerufen.
Someone from the office called earlier.