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.

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.
the
'die' and 'eine' can both stand before a feminine noun in the singular. 'die' is definite: the person or thing is known or clear. 'eine' is indefinite: the person or thing is new, any one of its kind, or not yet known in more detail. So you can say 'die Ärztin' (the doctor) if both people know which doctor is meant, and 'eine Ärztin' (a doctor) if you only mean some doctor. Learners confuse the words because both fit feminine nouns. Pay attention to whether you mean something specific or are introducing something new.

Hannes hat am Montag eine Besprechung beim Terminplaner eingetragen.
On Monday, Hannes entered a meeting in the scheduling system.
Sprich am Wochenende mit der Ärztin über die neue Medizin.
Talk to the doctor about the new medicine over the weekend.