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Enter and move down

Windows shortcut 
Enter
Mac shortcut 
Return

This isn't really a shortcut, but it's important to understand that pressing enter or return will complete a cell entry and move the cursor down by one cell. Using other keys (like the tab and arrow keys) you have the option to move the cursor in other directions after completing the entry in a cell.

Related videos

The videos below demonstrate this shortcut.
Setting up a running total in an Excel table is a bit tricky because it's not obvious how to use structured references. This video explores some options.
In this video, we'll look more closely at calculated columns in Excel Tables, a feature to automatically enter and maintain formulas.
In this video, we introduce you to the function. Excel ships with hundreds of functions for special purposes.
In this video, we look at how to use VLOOKUP to lookup values in an Excel Table.
In this video, we introduce you to the formula, the method by which you define a calculation.