When the EUR interest rate increases while the USD interest rate remains constant, the forward rate, EUR/USD is expected to decrease. This relationship is explained by the interest rate parity theory, which suggests that the difference between the forward exchange rate and the spot exchange rate should be equal to the interest rate differential between the two currencies.

In this scenario, as the EUR interest rate increases while the USD interest rate remains unchanged, the interest rate differential between the two currencies widens. To ensure interest rate parity, the forward rate for EUR/USD must decrease.

Mathematically, this concept can be illustrated using the covered interest rate parity formula:

$F = S \times \frac{1 + r_{\text{USD}}}{1 + r_{\text{EUR}}}$

As the interest rate of the EUR increases, the denominator in the formula will increase, leading to a decrease in the forward rate.

This relationship is crucial for investors and businesses involved in currency trading and hedging, as it helps them understand how changes in interest rates can impact the forward exchange rates of different currency pairs.