In the following sentence
I'm sitting in my chair and waiting for a dinner.
my daughter's teacher crossed out "a". What is the reason/rule here that I am waiting for dinner, but I am waiting for a train?
In the following sentence
I'm sitting in my chair and waiting for a dinner.
my daughter's teacher crossed out "a". What is the reason/rule here that I am waiting for dinner, but I am waiting for a train?
Dinner is both countable and uncountable. In case there is no adjective, it's treated as uncountable, therefore it should not have a preposition 'a' before it. E.g.
I'm waiting for dinner.
I'm waiting for a tasty dinner.
Source: https://forum.wordreference.com/threads/uncountable-nouns-can-be-countable-when.2262653/
Also when dinner means a formal social occasion, it's countable
They held a dinner to celebrate their company's anniversary.
Source: https://dictionary.cambridge.org/pl/dictionary/english/dinner
Context could be the issue here. Is she sitting at home waiting for dinner - in which case the omission is correct, or on a plane waiting for 'a' dinner of which she could be receiving one of a selection...
There is no rule. The expression
“waiting for dinner”
without the article is just usage which native English speakers imbibe with their mother‘s milk. (Likewise for other meals.)
You can see the relative usage in this Google ngram.
English is easy in that nouns lack genders or declensions. It is difficult in so far as sentence structure and article usage often deviates from the ground rules and preposition choice is a minefield. Learn it and live with it.