Intermediate Sudoku Techniques
At the intermediate level, you need techniques that focus on relationships between candidate numbers. Instead of looking at single cells, look at how cells relate to each other.
Naked Pair
Two cells share the same two candidates → eliminate from other cells in the unit
Pointing Pair
A number in a block is confined to one row/column → eliminate from other blocks
Box-Line Reduction
The reverse of Pointing Pair. A number in a row/column is confined to one block.