1. He works at/in the library.

2. He works at/in the town hall.
3. He works at/in the hospital.

(Which preposition do I have to use? Are both OK? Which one is popular?)

4. I want to drink iced tea.
5. I want to drink ice tea.
6. I want to drink an iced tea.
7. I want to drink an ice tea.

(Which ones are correct?)

well in first 3 both are ok but "IN" is more popular.

5. I want to drink ice tea.

I would disagree with Elvira, favoring 4 over 5.

"Iced tea" would indicate tea with ice in it, while "ice tea" suggests cubes of frozen tea to me.

6 is also good, but it indicates a limitation of one serving or glass of iced tea. It is just more specific than 4.

I hope this helps a little more. Thanks for asking.

Yes, PsyDAG is correct about the difference between "iced" and "ice" in this phrasing.

1. He works at the library.

2. He works at the town hall.
3. He works in the hospital.

In general, when talking about someone's place of work, either "at" or "in" can be used. However, the choice of preposition can depend on the specific context and meaning you want to convey.

"Works at" is commonly used when referring to a specific location or organization where someone is employed. It implies a physical presence at a particular workplace. For example, "He works at the library" means he is employed by the library and is expected to be physically present there during working hours.

"On the other hand, "works in" is used when someone is employed within a larger entity or industry. It implies a broader scope or association. For example, "He works in the hospital" means he is employed within the hospital industry or environment, but not necessarily at a specific location within the hospital.

Regarding your second question:

4. I want to drink iced tea. - This is the correct sentence. "Iced tea" refers to tea that is chilled or served over ice.

5. I want to drink ice tea. - This is not correct. "Ice" is a noun here, so it doesn't work to describe the type of tea.

6. I want to drink an iced tea. - This is also correct. "Iced tea" is used as a noun phrase here, indicating you want a specific beverage that is chilled or served over ice.

7. I want to drink an ice tea. - This is not correct. As mentioned before, "ice" is a noun, so it doesn't work to describe the type of tea.

To summarize, you can use "at" or "in" to refer to someone's place of work, depending on the specific context and meaning you want to convey. It is more common to use "at" when specifying a particular location, and "in" when referring to working within a larger entity or industry. In terms of the tea question, "iced tea" is the correct phrase to use.