Decade的概念： 從曆法的單位上來看，“decade”是曆法在十年的一個單位，同樣的還有lustrum （5 年），century （100年），millennium（1000年）. In the past decade的同義字'in the past decade' means that it happened at some point within the last ten years. I guess the only minute difference would be how we use them.
In a decade = in 10 years time this has no specific date.
In the/this decade = in these 10 years this implies you're talking about the current decade. I mean we can say two decades or three decades to. 'over the past decade' means that it happened repeatedly, more than once, or.
