Put exactly one tent for every tree; therefore the total of the row clues, the total of the column clues, and the number of trees must all be equal.
A tent is paired with a tree in the cell immediately above, below, left, or right of it. Diagonal adjacency does not pair a tent and a tree.
The completed board must have a one-to-one pairing: each tree is paired with one tent and each tent with one tree. A tent may stand next to several trees, and a tree may have several adjacent tent candidates, provided that a complete one-to-one pairing exists.
Two tents may not touch, horizontally, vertically, or diagonally. The eight surrounding cells of a tent cannot contain another tent.
The number at the end of a row or bottom of a column is the exact number of tents in that line. A 0 is a real clue and means the entire line has no tents.
Trees are fixed terrain, are not tents, and can never be edited or covered. A tree may be diagonally adjacent to a tent or adjacent to another tree.
The puzzle is solved when all tents are placed and all rules above hold. Marking every remaining cell as grass is optional and is not required for completion.
A row or column clued 0 holds no tents at all. Grass every one of its cells before you look at anything else — it costs nothing and it usually decides several neighbouring lines for you.
Example: Row 4 is clued 0. Mark all of row 4 as grass; any tree that was relying on a cell in that row now has one fewer candidate.
A tent must sit directly above, below, left or right of a tree. A cell with no tree on any of those four sides can never hold one, no matter what the clues say — diagonal neighbours do not count.
Example: A cell has trees only at its two diagonal corners. It can never be a tent, so mark it as grass.
Count the cells in a line that could still take a tent, then compare that with what the clue still needs. When the two numbers match, every one of those cells is a tent.
Example: A row is clued 3, has no tents yet, and only three of its cells are still candidates. Pitch a tent in all three.
Once a row or column holds its clued number of tents, everything else in that line is grass. The clue turns green the moment the count matches, so sweep the rest of the line straight away.
Example: A column clued 2 already shows two tents. Grass every remaining cell in that column.
Tents never touch, not even at a corner, so placing one rules out all eight cells around it. Marking that halo immediately is what turns one placement into the next deduction.
Example: After pitching a tent, grass each of the eight surrounding cells; a neighbouring row often becomes an exact fit as a result.
Every tree needs a tent of its own. If grass has ruled out all but one of a tree’s four neighbours, that last cell must be its tent — even when the line clues would still have allowed something else.
Example: A tree has grass above, below and to its left. Pitch a tent in the cell to its right.
Look at a small group of trees and the cells they could use. When two trees share exactly two candidate cells, both of those cells are tents, and nothing else in reach of that pair can be one — the two trees have used them up.
Example: Two trees can each only use the same two cells. Both cells are tents, so grass every other cell touching either of them.
Every tent belongs to a row and a column at once. After a placement, recount both — a line that has just become full or just become an exact fit is where the next certain move usually is.
Example: A new tent completes its row. Grassing the rest of that row leaves a column with as many candidates as it still needs, so all of them are tents.