Use the Sales Forecasts form to track sales forecasts.
When you create a new sales forecast, the grid at the bottom of the form shows all non-closed opportunities for the salesperson where the Include In Forecast field is selected on the Opportunity Statuses form and only those forecasts with a projected close date within the current period.
If you change the sales period, the opportunities displayed in the grid adjust to include only those with a projected close date within the new period. If the forecast has a status of Draft, then sales forecast opportunities are editable, and changes are made to both opportunities and sales forecast opportunities when you save the record.