ngôi

1. Respected Buildings and Communal Spaces
Use ngôi for buildings and spaces that hold emotional, cultural, or social significance. For example, ngôi nhà feels more like a valued "home" compared to the purely physical cái nhà. It is also standard for schools (ngôi trường), temples (ngôi chùa), churches (ngôi nhà thờ), and shrines (ngôi đền). Furthermore, ngôi extends to valued communal spaces like villages (ngôi làng) and markets (ngôi chợ). Note that purely functional, commercial, or administrative buildings (like bệnh viện - hospital, or nhà hàng - restaurant) generally take no classifier or just cái in casual speech. However, ngôi can indeed be used for these buildings in formal, historical, or literary contexts to convey respect or significance (e.g., ngôi bệnh viện cổ). If you want to emphasize their massive scale, you would use tòa instead.

2. Stars and Graves
Beyond buildings and spaces, ngôi is the standard, everyday classifier for stars (ngôi sao) and graves or tombs (ngôi mộ). Note that ngôi sao is also the standard compound noun for a celebrity or "star" in the entertainment and sports industries (e.g., ngôi sao điện ảnh - movie star).

3. Other Meanings
You might also see ngôi used as a regular noun meaning "throne" (ngôi vua), "grammatical person" (ngôi thứ nhất - first person), or for titles, ranks, and positions in sports and competitions (e.g., ngôi vô địch - championship title, ngôi á quân - runner-up position, ngôi đầu bảng - top of the table).

4. How it compares to other classifiers

• Ngôi vs. cái: Cái nhà is casual and focuses on the physical structure. Ngôi nhà is more respectful and literary.
• Ngôi vs. căn: Both can be used for complete houses. However, căn nhà is a neutral, objective way to refer to the physical housing unit (and căn is also used for apartments and rooms). Ngôi nhà is more literary, emotional, or respectful, focusing on the home as a valued space.
• Ngôi vs. Tòa: Use tòa for massive, imposing structures like skyscrapers. However, ngôi is perfectly acceptable for mansions and villas (ngôi biệt thự or ngôi dinh thự).

Remember, you must use a classifier when using numbers (một ngôi nhà). When using demonstratives like này, classifiers are usually used in formal speech (ngôi nhà này), but they are often dropped in casual everyday speech (nhà này). If you are not explicitly counting or pointing out a specific building, the classifier is also often dropped entirely (e.g., Nhà tôi rất lớn is more common than Ngôi nhà của tôi rất lớn).