Questions & Answers about لا أعرف اسم هذه الجارة، لكنها تسكن فوق شقتي منذ شهر.
Because الجارة means the female neighbor. The masculine form is الجار. Since the sentence is talking about a woman, Arabic uses the feminine noun.
هذه is the demonstrative for a feminine singular noun: this.
هذا is used with masculine singular nouns.
So:
- هذه الجارة = this female neighbor
- هذا الجار = this male neighbor
The demonstrative has to match the gender of the noun.
In standard Arabic, when a demonstrative like هذا / هذه is followed by a noun, that noun is usually definite, so it normally takes الـ.
So Arabic prefers:
- هذه الجارة = this neighbor
rather than:
- هذه جارة
That second version is not the normal MSA pattern for this neighbor.