>>217
What do you think you should use? Here is my opinion:
1) most likely "a chair," you are emphasizing that you are going to sit down rather than emphasizing that you are sitting in a special chair
2) probably "the chair." Here you are emphasizing that you are going to sit in the only chair.
>>219
It is useful to realize and to think about the fact that the definite articles and the indefinite articles mean different things.
I haven't been to a cinema in a while
vs
I haven't been to the cinema in a while
Each of these is grammatically correct, and each are used, but they mean different things. The first implies I haven't been to any cinema anywhere in a long time. The second implies we either use the same cinema, or we are talking about the one nearby. The never refers to something random, I may not be talking specifically about, say the NCG cinema in Grand Blanc, but I am talking about a certain one.
These sentences aren't as common as someone saying "I haven't gone to see a movie in a while" "I haven't gone to the theater in a while"
We (here, at least) don't really use the word cinema too often, we call them movie theaters are simply theaters. People don't watch too many plays here so there is no confusion as to what they mean.