1. J-Curve

2. Summary:
In the short term a depreciation of an exchange rate may not improve the current account deficit of the balance of payments
This is due to the low price elasticity of demand for imports and exports in the immediate aftermath of an exchange rate change. (Inelastic, at least in the short run)

4. When will the balance of trade improve?
Providing that the elasticities of demand for imports and exports are greater than 1 (elastic) in the long term the trade balance will improve over.
This is known as the Marshall-Lerner condition.

6. The Inverse J Curve Effect
An appreciation in the exchange rate can lead to a short term improvement in the balance of trade.
Imports become cheaper and exports more expensive in overseas markets.
But initially the elasticity of demand for both imports and exports is fairly low - leading to an overall improvement in the trade balance.