how can I create a form for entering data into a spreadsheet

It could be done with an Applescript designed specifically for your purpose. Your "form" would be a sheet with a table or set of tables and you would use a keyboard shortcut to submit the "form" to your "spreadsheet" table. The keyboard shortcut would invoke the Applescript to transfer the data and clear the "form" for the next entry.

If the "form" is limited to one table, it might be possible to create a more generic Applescript but it would take some work to write it. I am thinking it would involve a second table that tells the script where to submit the data (i.e., which column of the specified sheet::table) for each cell of the form.

If you are on a Mac then there are no forms. And forms aren't really needed because of the large screen size.

On iOS Numbers does have forms, which can be useful.

SG