The Wolfram System front end provides an Insert▶Table/Matrix submenu for creating and editing arrays with any specified number of rows and columns. Once you have such an array, you can edit it to fill in whatever elements you want.
The Wolfram Language treats an array like this as a matrix represented by a list of lists:
Putting parentheses around the array makes it look more like a matrix, but does not affect its interpretation:
Using MatrixForm tells the Wolfram Language to display the result of the Transpose as a matrix:
add a column
add a row
go to the next or element
move out of the table or matrix
Entering tables and matrices.
Note that you can use Ctrl+, and Ctrl+Enter to start building up an array, and particularly for small arrays this is often more convenient than using the New menu item in the Table/Matrix submenu. The Table/Matrix menu items typically allow you to make basic adjustments, such as drawing lines between rows or columns.
Entering a Piecewise expression is a special case of entering a table.
Enter the \[Piecewise] character and press Ctrl+, to get a template of placeholders for two cases:
Fill in the placeholders to complete the piecewise expression: