-았/었/였으면 좋겠다 is the grammatical structure that expresses a wish or a hope, and just like when the English verb 'to wish' is used, the conjugated structure of the verb in Korean is also usually in the past tense. The form in the present tense, -(으)면 좋겠다 can also be used, but the past tense is more commonly used, especially with the ending part “좋겠다”. 그가 왔으면 좋겠어요 = I wish he would come. it would be good if he would come. 그가 했으면 좋겠어요 = I wish he would do it. it would be good if he would do it. "it WOULD HAVE BEEN good if" translates to -았/었/였었으면 좋았겠다 it would have been good if he would have come. 그가 왔었으면 좋았겠어요