Lumber's a living thing. There will be warps. Here's how to avoid warped lumber or use the best available at the local big box hardware store.
Bowed:
Spotting a bow is done by holding one end of lumber up with the other down on a flat surface and rolling the lumber from corner to corner in a circle while sighting the broadest face.
This is easy to use if the curve isn't extreme; best practice is to fasten lumber when it can't be sawed-off before the warp. Bowed lumber is not ideal for wall studs or anything held up, but securing warped lumber to a straight length with a screw or nail will make the project sturdier.
Crooked:
Where the bow curves on the broad face, crooked lumber curves on the slim side. Same as the bow, roll from corner to corner while sighting the smaller face of the lumber.
Cut crooked lumber into smaller pieces. If lumber with a crooked warp is all that's there to buy, use more fasteners with a smaller distance between each screw or nail as available.
Twisted:
Roll the lumber, and watch the slim and broad face for warps in both. Twisted lumber is difficult to use. The best practice is to cut into smaller pieces than the crooked lumber.
The best practice for fastening warped lumber with screws or nails is skew-nailing. Traditionally skew-nailing is done at a 45-degree angle, but in practice, any degree is better than none. Lumber exposed to temperature and moisture can contract or expand fasteners will exit at the same degree inserted.
Bowed:
Spotting a bow is done by holding one end of lumber up with the other down on a flat surface and rolling the lumber from corner to corner in a circle while sighting the broadest face.
This is easy to use if the curve isn't extreme; best practice is to fasten lumber when it can't be sawed-off before the warp. Bowed lumber is not ideal for wall studs or anything held up, but securing warped lumber to a straight length with a screw or nail will make the project sturdier.
Crooked:
Where the bow curves on the broad face, crooked lumber curves on the slim side. Same as the bow, roll from corner to corner while sighting the smaller face of the lumber.
Cut crooked lumber into smaller pieces. If lumber with a crooked warp is all that's there to buy, use more fasteners with a smaller distance between each screw or nail as available.
Twisted:
Roll the lumber, and watch the slim and broad face for warps in both. Twisted lumber is difficult to use. The best practice is to cut into smaller pieces than the crooked lumber.
The best practice for fastening warped lumber with screws or nails is skew-nailing. Traditionally skew-nailing is done at a 45-degree angle, but in practice, any degree is better than none. Lumber exposed to temperature and moisture can contract or expand fasteners will exit at the same degree inserted.