How should you open a file preview window after selecting a file in the Finder?

To open a file preview window in Finder, pressing the Space bar is the correct method. This action triggers Quick Look, a feature that allows you to quickly view the contents of a file without fully opening it in its associated application. Quick Look provides a convenient way to examine files, such as images or documents, enabling you to make decisions about what to do with the file next—such as opening it completely or moving on to another file.

Double-clicking the file typically opens the file in its native application, which may not be efficient if you only want to quickly glance at its contents. Pressing the Enter key generally renames the selected file rather than opening a preview. While right-clicking and selecting 'Preview' would also allow you to see the file, this is a more time-consuming method compared to the simple and instantaneous action of pressing the Space bar. This makes the Space bar the most efficient choice for quickly previewing files in Finder.