Ringing tones
Your violin has a tuner built into it, and it's free. When a stopped note is exactly in tune with an open string, that open string starts to vibrate on its own — a faint, warm hum under the note you're playing. Sympathetic resonance. When you hear it, the note is right.
The 3rd-finger octave
The clearest one: your 3rd finger on the A, D and E strings plays the octave of the next open string but one below — and rings it.
The 1st-finger octave
Your 1st finger on the G and D strings does the same thing with the string two above:
Which notes ring
- A string: 3rd finger (D) rings the D string · 4th finger (E) matches the open E
- D string: 1st finger (E) rings the E string · 3rd finger (G) rings the G string · 4th finger (A) matches the open A
- G string: 1st finger (A) rings the A string · 4th finger (D) matches the open D
- E string: 3rd finger (A) rings the A string
The usual mistake
Not listening for it. The ring is quiet, and until you know to expect it, it slips past. Play the note, then stop moving and listen into the sound for a second.
Try
Play a slow G-major phrase — G, B, D, E, D, B, G on the G and D strings. As each note lands, hold it and listen for a string humming somewhere else. The D and G especially should light up their open strings.