How do I load another xaml file in tab content or a grid?

Is another test.xaml in the same solution as mainwindow.xaml? Also, is test.xaml a user control? If yes, then at the top declare: xmlns:view="clr-namespace:(project your test.xaml is in);assembly=(project your test.xaml is in) then inside your mainwindow.xaml in the tab control or grid simply do <Grid> <view:test/> </Grid> If you can give me a little more to go on I feel sure I can point you in the right direction.

This is an example of what it should look like for referencing a user control in your main window:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:view="clr-namespace:MyTestProject;assembly=MyTestProject"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <view:Test />
    </Grid>
</Window>