要加a,如果说是long long ago(也是很久以前的意思) 就不用加a
a long time ago:很久以前
例句与用法:
1.I'm afraid I can't remember it was such a long time ago.
很抱歉,我想不起来了--那是很久以前的事了.
2.A long time ago the tiny country of Switzerland was ruled by Austria.
很久以前,瑞士这个小国被奥地利统治.
3.A long,long time ago,cowboys were wanderers,driving their herd from place to place,looking for fresh grass and water.
在很久很久以前,牧童都是游牧者,赶着他们的牧群到处寻找青草和水源.
4.I forgave her a long time ago.
我早已原谅她了.