Isaiah was preaching to exiles about coming home. Mark told about a man who when cured of his blindness chose to follow Jesus on the way.
Psalm 126 also speaks of both what has been and what is to come.
They had lost their possesions. They had sorrowed. They had weeped. And, now, things will change.
The first stanza, verses 1-3, are set in the past. The Lord has already done these things that we rejoiced about. The second stanza, verses 4-6, however, are a prayer for the restoration of fortunes, a plea that these good things will happen.
That is, although we have had opportunity to say thank you, we now need once more to say please.