Solution: The Theft in the Potting Shed in the Gardens

The clues

• 
  LenaThe painter who never finishes a canvas.

  She was in the second row.

• 
  Dario · VictimThe jazz musician.

  The victim. He was alone with the killer. He was on a bench.

• 
  KarimThe family lawyer.

  He was on a bench. He was beside a well.

• 
  FelixThe student who never sleeps.

  He was in the Courtyard. He was beside a bench.

• 
  MateoThe waiter who hears everything.

  He was on a bench.

• 
  SofiaThe chess prodigy.

  She was in the Vegetable Garden.

• 
  HugoThe retired colonel.

  He was on a chair.

Step by step

The order our solver finds. Each step uses only the clues and what the steps before it proved, and its heading links to the technique.

1. Last cell left Every other cell is ruled out for Karim, by his testimony or by a row or column already taken. Placed: Karim: row 6, column 3 (Patio).
2. Pinned suspects Lena can only be in the second row, so nobody else can be there.
3. Room counting The victim, Dario, cannot be in the highlighted cells: his room there could not hold exactly one other person. The cells meant: row 5, column 6.
4. Shared rows and columns Felix, Sofia and Hugo can only be in the fourth row, the fifth row and the bottom row between them, so nobody else can be in those rows.
5. Shared rows and columns Dario and Mateo can only be in the left column and the fifth column from the left between them, so nobody else can be in those columns.
6. Pinned suspects Sofia can only be in the fifth row, so nobody else can be there. Placed: Felix: row 4, column 6 (Courtyard); Hugo: row 7, column 2 (Patio); Sofia: row 5, column 4 (Vegetable Garden); Lena: row 2, column 7 (Terrace).
7. Room counting The victim, Dario, cannot be in the highlighted cells: his room there could not hold exactly one other person. The cells meant: row 1, column 1. Placed: Dario: row 3, column 5 (Terrace); Mateo: row 1, column 1 (Gazebo).