-da

Explanation

1. -da
A connective ending used when an action or state, etc., is stopped and changed to another action or state.

2. -da
A connective ending used when something happens while a certain act is ongoing.

3. -da
A connective ending used when the preceding statement is the cause or reason for the following statement.

4. -da
A connective ending used when more than two facts happen in turns.