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fromatto:It was so boring that I left halfway through.
Why must use "through" here? Can it just say " I left halfway"? Please.
It's halfway through the total duration of the lecture.
If you say 'I left halfway' an English listener will be waiting for you to say 'through'. I suppose in extremely casual English you could 'get away with' saying 'I left halfway', but it doesn't sound right.
middle 
Linear Mode
