The head locks anywhere along the blade's length so you can mark a line parallel to a board's edge by sliding the head along the board. When marking lines, slide the combination square to your mark, then complete the line with a sharp pencil or scribe. For cutting wall studs and plywood, most people use a circular saw. A handsaw is the best answer where there isn't room for a circular saw or when you're far from a power source. These days, most carpenters rarely use a handsaw unless they have to.
Hacksaws feature replaceable, hardened blades with fine teeth.
Thinner metal, finer teeth. Better ones have sturdy frames and use levers rather than wing nuts to tighten the blade. Some can hold a blade at 45 degrees to facilitate flush cutting. Look for bimetal blades. Better still, choose a knife that doesn't require a screwdriver for blade changes.