girlfriend
The word 'Freundin' in German is used to refer to a female friend or a girlfriend. It is a noun and its gender is feminine. It is important to note that in German, the word for 'girlfriend' and 'female friend' is the same, so context is important to understand the meaning. For example, 'Sie ist meine Freundin' can mean either 'She is my girlfriend' or 'She is my female friend'.
My girlfriend is buying a book.
This sentence explains an action performed by 'my girlfriend'. Here, 'Freundin' is used to refer to a romantic partner.
Paul's girlfriend is very pretty.
This sentence gives an opinion about 'Paul's girlfriend'. Again, 'Freundin' refers to a romantic partner. The structure 'von Paul' is used to denote possession, similar to the english 'Paul's'.
The girlfriend can sing very well.
This sentence is speaking about 'the girlfriend' and her ability to sing well. The grammatical structure uses 'die' to speak about a specific girlfriend in this sentence.