1. Bend down, then I'll jump over you.
2. Bend down, and I'll jump over you.
3. If you bend down, then I'll jump over you.
---------------------
Is #1 grammatical?
You need a semi-colon after down.
Bend down; then I'll jump over you.
2. Bend down, and I'll jump over you.
3. If you bend down, then I'll jump over you.
---------------------
Is #1 grammatical?
Bend down; then I'll jump over you.