keep getting "Improper Statement" errors

I've spent hours trying to do a simple IF statement and I keep getting errors. I'm on the trial version and attempting to setup a quick demo for a customer so that they can decide if Zoho works for them.

It's a simple Commission tracking application. There are Sales Reps, Expense Categories, Customers and Invoices.

On the Invoice, there are multiple Expenses (subform). Based on the Expense Category of the Expense, I need to pull in the Rate_Type and calculate the amount of the expense.

It seems that everything I do I get an "Improper Statement" error.

expense_row = Expense_Categories [ID == Invoice_Expenses.Expense_Category];

if(expense_row.Rate_Type == "Variable" )

{Invoice_Expenses.Quantity == input.Quantity};

else if(expense_row.Rate_Type == "Fixed" )

{Invoice_Expenses.Quantity = 1};

else if(expense_row.Rate_Type == "Formula" )

{Invoice_Expenses.Quantity = input.Quantity};