Shouldst is an old form of should, used when you are talking to one person with thou (the old word for you). Thou shouldst means you should. The meaning is not different from should; only the grammar costume is old. You will meet it in the Bible, in Shakespeare, in hymns, and in stories that want to sound like the past. Nobody uses it in ordinary talk now, except as a joke, in a play, or when someone is pretending to be a knight. If a sentence has you instead of thou, the form is should, not shouldst.

古體的 should（thou shouldst＝你應當）