Triggers collection members must be of type EventTrigger

From MSDN Docs, as per the (slightly paraphrased) answer from Richard C. McGuire:

DataTriggers can be used with XML tags Style, ControlTemplate and DataTemplate

For example, if you attempt to add a trigger to a TextBlock, it will generate this error:

Error: Triggers collection members must be of type EventTrigger

Why? A Trigger can only be placed inside a Style, ControlTemplate or DataTemplate, and we are trying to place it directly inside a TextBlock.

In this case, the fix is easy: just wrap the trigger in a style, then place this style inside the TextBlock, and the error will disappear.

Here is the error-generating XAML before the fix:

<TextBlock x:Name="Hello" Text="{Binding Hello, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
  <TextBlock.Triggers>
      <DataTrigger Binding="{Binding Hello}" Value="GoGreen">
          <Setter Property="Foreground" Value="Green" />
      </DataTrigger>
  </TextBlock.Triggers>
</TextBlock>

Here is the XAML after the fix:

<TextBlock x:Name="Hello" Text="{Binding Hello, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
    <TextBlock.Style>
        <Style TargetType="TextBlock">
            <Setter Property="Foreground" Value="Red" />
            <Style.Triggers>
                <DataTrigger Binding="{Binding Hello}" Value="GoGreen">
                    <Setter Property="Foreground" Value="Green" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

Here is a sample screenshot showing that if we enter GoGreen, the text goes green:

… and if we enter something else, the text defaults to red:

There is loads of free material on the web about WPF triggers, and all of them do a reasonably good job of explaining the concept, and this page was the one that made the penny drop for me.