问一个英文语法问题Time is but the stream I go a-fishing in为啥要加a?

问一个英文语法问题
Time is but the stream I go a-fishing in为啥要加a?

Spring
Spring,the sweet Spring,is the year’s pleasant king(1);
Then blooms each thing,then maids dance in a ring,
Cold doth not sting(2),the pretty birds do sing(3),
Cuckoo,jug-jug,pu-we,to-witta-woo!(4)
The palm(5) and may(6) make country houses gay,
Lambs frisk’ and play,the shepherds pipe all day,
And we hear aye birds tune this merry lay,(7)
Cuckoo,jug-jug’ pu,we,to-witta-woo !
The fields breathe sweet,the daisies kiss our feet,
Young lovers meet,old wives a-sunning sit(8),
In every street these tunes bur ears do greet(9),
Cuckoo,jug-jug,pu-we,to-witta-woo!
Spring!The sweet Spring!
春天
春天,甜蜜的春天,一年中欢乐的君王,
春光中万物昌盛,春光中少女们团团起舞,
寒冷不再砭骨,美丽的鸟儿处处啼唱,
咕咕,喳一喳,卜一威,嘟一威嗒一喔!
棕榈树和山楂花把乡村房舍点缀得喜气洋洋,
小羔羊活蹦欢跳,牧羊人整日价笛声袅袅,
我们时时斯到鸟儿唱起快乐的曲调,
咕咕,喳一喳,卜一威,嘟一威嗒一喔!
田野间一片芬芳,盛开的雏菊紧贴在我们脚旁,
年轻的情侣对对相聚,老妇人坐着沐浴阳光,
每条街上都有这样的曲调在我们耳边回荡,
咕咕,喳一喳,卜一威,嘟一威嗒一喔!
春天!甜蜜的春天!
托马斯·纳什
(8)old wives a-sunning sit：sit是谓语动词.a-sunning：晒太阳,作状语; a-作为特殊介词,在现代英语中已不如此使用.例如：在现代英语中不说He went a-fishing而说He went fishing.
Time is but the stream I go a-fishing in.I drink at it; but while I drink I see the sandy bottom and detect how shallow it is.Its thin current slides away,but eternity remains.
徐迟译文是：
时间只是我垂钓的溪.我喝溪水,喝水时候我看到它那沙底,它多么浅啊.它的汨汨的流水逝去了,可是永恒留了下来.