Select the row below where you want the new line
Right-click the selected row
Click Insert
To insert multiple lines, select multiple rows first
Use the Home tab
Click Insert in the Cells group
Choose Insert Sheet Rows
Use the keyboard shortcut Ctrl + Shift + +
On a Mac, use Command + Shift + +
To insert a line within a cell, click the cell
Press Alt + Enter on Windows
Press Control + Option + Return on Mac