0I would say at the theatre or more specific at the movie theatre or at the movies. Regarding the definite article, it's often used because it sort of refers to the place that you always go or the nearest one, for example, I went to 01i 00the 02i 00beach, I was at the post office, I was at the bank. However, you can say at a park because there's usually many parks in a town.
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01cite10anglista200812cite10Hey again,12br
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10More preopositions. This time - would a native speaker rather say:12br
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101) I was AT or IN the theatre yesterday.12br
102) I was AT or IN the cinema yesterday.12br
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10And do I HAVE to use the definite article