A good Inkscape tool is the Pen. It lets you draw Bézier curves and straight lines. Understanding node types is everything. This post gives you a feel for it.

1 Node Types

Pay attention to the node types when drawing: smooth and symmetric ones are conveniently used for a large variety of purposes, except for those few select cases where you do need corners, for which there is no substitute for corner nodes. When drawing, it's not clear how Inkscape decides between smooth and corner nodes. It's probably vaguely related to how far handles are pulled.

Also, boolean operations may cause smooth nodes to become corner nodes. But that isn't a problem per se as the curve itself will not change. If you need to work with it again, just convert it back to a smooth node and you're good to go.

2 Spiro Paths

Even if they are presented as a particular mode to change the Pen's primary function, Spiro paths are nothing more than paths which have been applied the Path → Path Effect Editor → Spiro spline effect. As a result, if you drew a Spiro path by mistake when in fact you meant to create a regular one, you can always Path → Remove Path Effect. Spiro paths make for round paths which are useful for organic subjects.

3 Shapes

Shapes are simply applied as they would be with Path → Path Effect Editor → Pattern Along Path. The Pattern path is most conveniently used with links, which are a little odd to create: you're supposed to copy to the clipboard the object you want to link to and press the Link to path button. It may feel like you're linking from a copy, but in fact you do link from the original object – no actual copy made here.

Be careful when specifying the shape with From clipboard in the Pen tool: this will assume a true copy, no link. Objects suffering from this are best tracked in the XML file by looking for inkscape:path-effect tags whose pattern refers to another path (e.g. pattern="#path5107"), rather than an in-line collection of static nodes.