SUMIF across sheet

Hello, this feels like it should be simple but I'm having trouble making it work.  I have a a workbook with two sheets (tabs).  In the first tab, I need to create a formula to look into the second tab, which is an amortization table.  I need to have Excel look down a column in the amortization table that contains dates.  If a certain date is found, I then need Excel to look across the the same row in the amortization table to a column showing the amount paid that month.  I've tried entering the "+" sign and then selecting the column in the second spreadsheet with the dates, then entering the date, then telling it to sum_range for the associated range in the column for the payment.  However, the formula doesn't return anything and thinks something's wrong.  I've browsed the web and am not finding anything that works.  What am I doing wrong?  Thanks much.