Please watch this video or follow the steps below:
1) Click "Activities".
2) Click on the "All Activities" dropdown and select "Tasks".
3) Click on the "Status" dropdown and select "Not Completed"
4) Click on the "Assigned To" dropdown and select the desired user.
You can also search for incomplete tasks throughout a certain date range by clicking on the "Date Range" dropdown and selecting the dates in "Custom Date Range".
What happens now?
Your tasks list will now automatically update to show you tasks that have not been completed by the specified user.