There are three hint level available.

Level 0

No hints. All possibilities are available.

Level 1

Only numbers absent in row, column and square are displayed to click on them.

Level 2

Program performs more complex board analysis and eliminates more numbers than in level 1.

Highlight lonely numbers

This option is available in levels 1 and 2.

Error

Sometimes, in level 1 or 2, you will see empty, pink squares. This means that there is no number to put in. You should press Back and try another move.