What counts as a win
The game checks the whole board after every mark. You win the moment four of your marks sit side by side in a straight line: across a row, down a column or along either diagonal. There are 54 such lines on the board (18 across, 18 down and 18 on diagonals). In the pictures on this page you are X (cyan) and the computer is O (magenta), as in the game. Cells are named by row, then column, counted from the top left.
Some shapes look close but do not win. Three in a row is only a threat, a gap breaks a line, and an O in the middle ends it. If all 36 cells fill up with no four in a row, the round is a draw.
Why the middle of the board matters
Each number below is how many of the 54 possible lines of four pass through that cell. A mark on one of the four central cells sits on 11 lines. A mark on an edge sits on 4 or 5, and a corner mark sits on only 3. More lines means more chances to build a threat, and it also means a mark there ruins more of your opponent's lines.
That is why a central opening is a good habit, and why a corner mark usually does the least. It is a guide, not a rule: a corner can still be the right move when it finishes or blocks a line. For how the computer weighs positions, see the AI notes on the Neon Tac page.
Threats: how three in a row can be stopped
A line with three of your marks and one empty cell that would make four is a threat: if you get to play that cell, you win. What matters is how many cells finish it, because your opponent has one move to answer.
So an open three wins unless the other side can win first. Block a three early, while it is still two in a row with room on both sides, because once it is open you can no longer stop it with one move.
Two threats at once
The same idea works across different lines. If one mark gives you two different cells that each finish a line of four, your opponent can only fill one of them. An open three is the simplest way to do it. The game page notes that even the Hard level can be beaten this way.
Practice positions
Neon Tac always starts a round from an empty board, so these positions cannot be loaded into it. Use them as exercises here, then play a round and try to build the same shapes. In each one you are X and it is your move, and both sides have the same number of marks. The answers are hidden until you open them.
Finish the line
You are X and it is your move. One cell wins at once. Which one?
Show the answer
The diagonal from the top-left corner already holds X on the second, third and fourth squares. Playing the corner makes four in a row. The magenta O on the other end has closed that side, so there is only one winning cell.
Block the threat
You are X and it is your move. O will win next turn unless you stop it. Where do you play?
Show the answer
O has three in the third column starting at the top edge, so the only way to finish four is the cell just below them. You have no win of your own, so block it.
Make two threats at once
You are X and it is your move. Which cell leaves you with two different winning cells, so that O can only block one?
Show the answer
Playing row 3, column 4 gives X three in a row (columns 2 to 4) with an empty cell at both ends. Column 1 and column 5 of that row each make four, and O can take only one of them.
Every position above was checked with the same win rule the game uses: the first has exactly one winning cell, the second has exactly one cell the computer would win with, and in the third no other cell gives you two threats at once.
Play it for real
In Neon Tac you place marks by clicking or tapping a cell, or by moving the highlight with the arrow keys and pressing Enter or Space. Easy shows the cells where the computer could win next, which is a good way to train your eye for threats. Related logic games are Minesweeper and Sudoku.