Edit Menu

Undo Undo the last operation.
Cut Cut selection to the clipboard.
Copy Copy selection to the clipboard.
Paste Paste the clipboard.
Select_None Deselect all.
Delete Delete the selection.
Duplicate Duplicate the selected object. Objects can also be duplicated by holding the [Shift] key and dragging, or by hitting the [Ins] key while dragging.
Realign to Grid

Realign the selection to the grid, according to the World, Local, or Delta Snap mode selected in the toolbar Snap Tool.

Hollow Block This function only applies to blocks, not groups. It hollows out a solid block, breaking it down to multiple thin blocks surrounding an empty area inside. This is often used to create rooms out of enlarged cubic blocks.
CSG Subtract This function only applies to blocks, not groups. Works very much like a cookie cutter. It carves holes out of solid blocks using the selected object as the cutting shape. It removes the portions of other blocks intersecting the selected object. The texture of the cutter is assigned to the cut planes. This function is often used to 'chop off' pieces from blocks, or to break open the wall inbetween adjacent rooms to make a doorway.
Object Mode

Only certain object types can be selected and changed.

All Objects All objects can be selected and changed.
Group only groups can be selected and changed.
Block only blocks can be selected and changed.
Entity only entities can be selected and changed.
Start only positions can be selected and changed.
Light only lights can be selected and changed.
Model only models can be selected and changed.
Sound only sounds can be selected and changed.
Path only paths can be selected and changed.
Find Object Finds a block, entity, or texture by its number or the beginning of its name. Invalid or untextured blocks, which have caused a warning or error message in the Build process, can also be found this way. If the object was contained in a group, the whole group will be highlighted; you can Scope Down and repeat Find Object to identify the block.