If the interest rate of currency A is 10% and the interest rate of currency B is 20% (although the interest rate difference here is exaggerated, the interest rate of each currency is actually different, and there is an interest rate difference).

Spot exchange rate A=2B (for simplicity, the buying price and selling price are not considered here).

There is a possibility: borrow 1A currency (the loan term is one year, and the deposit interest rate is the same as the loan), immediately convert it into 2B currency, deposit it (the deposit term is the same, and the interest rate is 20%), take it out with principal and interest after one year, and then convert it into currency to repay the principal and interest of the loan1a. If the exchange rate is unchanged for one year, there is no transaction cost in this process.

2 * (1+20%)/2-1* (1+10%) =1*10% = 0.1a

Because the currency spread exists, it is not done, because after one year, the principal and interest (forward) of B are taken out, and the exchange rate converted into A changes, and the market is in an equilibrium state. Theoretically, the equilibrium exchange rate R can be calculated.

2 *( 1+20%)/R = 1 *( 1+ 10%)，R=2. 18 18

Obviously, R has increased, that is, the currency B has depreciated (the interest rate parity of economist Keynes mainly refers to the forward depreciation of the currency with high spot interest rate).

Theoretically, when the forward exchange rate (here, one year) is less than 2. 18 18, it is profitable to convert low-interest currencies into high-interest currency deposits (arbitrage spread).

earn:2 *( 1+20%)/r- 1 *( 1+ 10%)> 0

When the forward exchange rate (in this case, one year) is less than 2. 18 18, it is profitable to convert high-interest currency into low-interest currency.

Revenue:1* (1+10%)-2 * (1+20%)/r > 0

I wonder if this example has solved your problem.