In all of your examples you can either use 'the' or omit 'the' depending on context and what you are trying to say. If the specific dinner or lunch is important then you would use 'the' eg Obama was the guest speaker at the dinner last night. eg The lunch you made me on my birthday was delicious eg After the dinner we had together, he decided to propose. eg After the lunch I felt sick. I think it was food poisoning If the specific lunch is not important then you can leave out the 'the'. ( the important part of what you are saying is not about the lunch) eg Please can you explain the work at lunch eg At dinner I need to show you my new shoes. The same principle applies to the hospital. If the specific hospital is important then you include 'the' if not it can be omitted. eg I am in hospital at the moment with a broken leg. eg I am going to be in the hospital for at least three weeks eg I am at the hospital near the park here however you wouldn't say'at hospital' you need to say 'at the hospital' OR 'at a hospital' This is the same for any place It can be specific eg. at the park eg at the supermarket or general eg at a park eg at a supermarket When saying 'in hospital' you are referring to an experience and not a place. You are describing the act of being treated in a hospital and therefore you can say i am in hospital without an article If you want to say that you are standing inside the hospital then you need the article again. eg I am waiting for you in the hospital.

The difference between at, in or on. At- refers to a place, a location or a state of being where you are currently. eg I am at the shops eg I am at peace eg I am at the museum eg I am at the beach eg I am at the mudflat In- refers to being inside something. eg I am in the pool eg I am in the museum (inside not just at the front door) eg I am in trouble On- refers to being atop something. eg I am on the stairs eg I am on a boat eg I am on the beach Because the beach has a different physical structure. If you are 'on the beach' you are literally on top of the sand of the beach. Final example. I am at the shop- I am standing where the shop is ( probably at the front door) I am in the shop - I am currently within the shop I am on the shop- I am standing on the roof of the shop

1.) We say just "at dinner/lunch, or after dinner/lunch". 2.) You can use both of them but as an example you would write it as: "She was in a museum". If you are at a museum, you'd say "I'm at a museum" depending on context! 3.) Again both are right! You need the 'the' though. 4.) The last ones are right. Just be careful with "on the beach". You can use this as: "She was laying on the beach." Hope I helped. Also if this clumps together I'll resend it detached!