Place light bulbs in the grid so as to light up all the blank squares. A light illuminates its own square and all the squares in the same row or column unless blocked by walls. Lights may not illuminate each other. Each numbered square must be orthogonally adjacent to exactly the given number of lights.
On a touch screen, a tap does what a click does, and a long press what a right-click does (more ways to right-click).
Click a square to place or remove a light. Right-click it to place or remove a cross, marking a square you think holds no light. Keep the right button down and drag to do the same to every square you pass over that looked the same as the first.
With the keyboard, the arrow keys move a cursor around the board. Enter does what a click does to the square under it, and Space (or I) what a right-click does.
A square holding a cross takes no light until you remove the cross (right-click it again), and a square holding a light takes no cross.
A light that gives a numbered wall all its lights rules out the wall’s other squares. With Cross out the other squares round a numbered wall once it has all its lights turned on in the preferences, those marks are made for you as you place: one Undo takes back the placement and the marks together, and taking the placement away later leaves the marks for you to clear.
Hint explains the next step rather than simply making it. Each step either puts a bulb in a square or, when a square “can’t hold a bulb”, marks it with a cross. The hint reasons from the numbers and your own bulbs and crosses, so it carries on from wherever you are, as long as none of them is wrong; if one is, it asks you to fix the highlighted mistakes first. A dark square, in its words, is a blank square no bulb lights yet; a free one is a blank square with neither a bulb nor a cross.
A few ideas are worth learning by name:
On an Unreasonable board the hint may stop and say that nothing further follows by deduction.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.