That-Agreement.
When the Principal and Subordinate Clauses are joined by the conjunction that:
1. A Past tense in the Principal Clause is followed by a Past tense in the Subordinate Clause, expressed by might or should.
I told him so that he might know.
He wrote it down so that he should not forget.
2. A Present Tense in the Principal Clause is followed by a Present Tense in the Subordinate Clause, expressed by may; or by a Future Tense.
The doctor gives us medicine that we may get well.
I hope that he will come.
3. A Future Tense in the Principal Clause is followed by a Present Tense in the Subordinate clause.
I will telegraph him so that he may know what to do.
We shall start early so that we may get there before dark.