怎样来区分定冠词和不定冠词a/an the
特别是特指
He only has one dictionary.他只有一本词典。（ 正确 ）
He only has a dictionary.他只有一本词典。（ 错误 ）
One minute is enough for me to do it .我做这件事一分钟足够了。（正确 ）
A minute is enough for me to do it .我做这件事一分钟足够了（ 错误 ）
为何错？