The Shape Builder Tool is a fast way to perform Pathfinder functions faster than using the Pathfinder Panel. It can add, subtract fills and strokes as well as breaking section apart. Proper set up to begin with is helpful to be able to get the results you want quickly and consistently.
Double click on the Shape Builder Tool in the Tool Bar to call up the options for the Tool. The options allow the shapes to be open or closed shapes when selecting and using the Shape Builder Tool.
The options will also allow you to split the paths of the shapes into sections that can be deleted or edited as a seperate shape.
Pick color option allows you to set the way the colors get merged. If the option is set to Artwork it will pick up the fill and stroke of the object that is being merged. Color Swatches option will blend the last used color (the ones that appear in the color picker) until you choose another color.
Start with shapes that have a stroke and/or fill. Select the shapes you want to merge or subtract. Use the Shape Builder Tool to select the area (shows a mesh pattern) and click. If the options are set to Color Swatches it will use the last used fill and stroke color to fill the selected shape.
By clicking on the shapes, it will add the new fill and stroke to those shapes.
The last used swatches in this example was orange file, red stroke.
To change the mode from ADD (+) to SUBTRACT (-) hold OPTION down to activate the Subtract moded
With the options set to Artwork mode, this will pick up the fill/stoke of the artwork. You can then click and drag across the shapes you want to add together.
To break a stroke apart, you can hover over a selected path and click to "break" it apart.
Then use the Group Selection Tool to move the object with out having to ungroup the set of objects.
Another option is to have the Shape Tool set to Straight line instead of Freeform to draw a connecting line between shapes
This is the tool in Freeform drawingmode.
Here's how you build a fish using the Shape Builder. Start with these basic shapes, select them all.
Add the tail to the body by dragging over both, adding them together.
Hold OPTION to subtract as you drag over the mouth.