Which Sandustry keys actually matter, and where do I rebind them?

Steps

1. Open Escape, Options, Controls once before you learn anything by habit. The full list is there, each entry can be reassigned, and a single action can carry more than one bind.
2. Learn delete and cancel first: hold X and drag to remove, right-click to abandon a placement you have started. Two channels demonstrate both, and one of them right-clicks twice to leave delete mode entirely.
3. Use F as a picker. Hold F and click any structure already in your world and your cursor is now building that structure. Two channels show this, and both describe it as faster than going back to the inventory.
4. Hold Control while placing to replace. Without it, a tile that already holds something will not accept a new building. This is the shortcut for upgrading a line to its Mark 2 version without demolishing it first.
5. Press R to cycle placement modes on any building that has them, then read the mode name shown on screen before you drag.
6. Reach for the menus by letter rather than by mouse: Q buildings, Tab toolbox, T research, U Fluxite upgrades, M map once it is unlocked.

Requirements / limits

• Delete (X), cancel (right-click), the picker (F), hold-Control replace and the R build-mode cycle are each shown by two or more independent channels. Those are the claims this page rests on.
• The following are shown by one channel only and are recorded here as single reports, not as verified binds: E opens a radial menu of recently used items, V is a ruler that counts tiles as you drag, Alt flips the direction of a diagonal foundation, holding B while dragging switches to free placement, Alt plus scroll or Alt plus 1 to 5 switches between the five hotbars, and Control plus Z undoes.
• Hover on Space is not available from the start. One recording states it has to be unlocked in the research tree first, and demonstrates it only after unlocking.
• The map on M is likewise gated behind an unlock in the same recording.
• Because every bind is reassignable, any key named here can differ on another player's machine. The Controls tab is the authority for your own install, not this page.
• No source publishes a complete key list in text form. This page names only keys that were pressed on camera with the result visible.

Common mistakes

• Demolishing a whole conveyor line to upgrade it. Hold Control and build the new version straight over the old one.
• Opening the inventory to place a block that is already sitting two tiles away. That is what F is for.
• Assuming a building only places one way because the first drag produced a line. Press R and look at the mode label before deciding a placement is impossible.
• Pressing Escape to get out of a half-finished placement. Right-click is the cancel; Escape opens the menu.
• Concluding that hover is broken because Space does nothing. It is a research unlock, not a default ability.

Still unverified

• Is there an official published list of default keybinds?Not verifiedThe in-game Controls tab is the only complete list anyone has shown. No publisher page reproduces it, so this page cannot claim to be exhaustive.
• Can a bind be set to a hold rather than a press?Single unconfirmed reportOne recording speculates on camera that a hold bind is probably possible while looking at the Controls tab, and does not try it. That is speculation, and it is recorded as such rather than repeated as fact.
• How many hotbars are there, and is five a cap or a current default?Single unconfirmed reportOne recording counts five and binds Alt 1 through Alt 5. No source says whether that number can change.