Keeping the fingers down
The left hand has a habit to build now, before it builds a bad one: fingers drop, and fingers stay. A hand that lifts every finger for every note — the "typewriter" hand — is slow, and it lands late and out of tune.
How a finger moves
It drops from the base knuckle, curved, landing on the tip near the nail — like a small hammer, straight down. Not the pad (the fleshy part lets the pitch wander, because the notes are so close together). Not a twist from the side.
Minimum pressure. The string only has to reach the fingerboard — that's a light touch, not a squeeze. If the base of your first finger or your thumb is clenching, the hand freezes.
And fingers stay down
When you play a higher note, leave the fingers behind it down on the string. Going back down the string, they're already there — no travel, no re-aiming.
The 1↔3 drill
Fingers 1 and 3, back and forth on the D string — keep the 2nd finger hovering just above the string the whole time, and keep 1 down when 3 goes on. This is Suzuki's "jumping finger".
The 2↔4 drill
Now fingers 2 and 4 — the weaker pair — with 1 and 3 planted underneath the whole time.
Blocked fingers pay off coming down
The other silent drill
One more, away from any tune: hold all four fingers down on the A string, then lift and tap one at a time — 1, 2, 3, 4, then 4, 3, 2, 1 — keeping the other three planted. It isolates each finger's independence without the bow.
The usual mistake
Lifting the whole hand off between notes, or lifting fingers you don't need to lift. Also pressing hard — a squeezed finger can't drop quickly and can't make the small in-tune adjustment.
Try
Left-hand pizzicato: with the bow down, pluck the A string with your 4th finger — a quick, firm little flick. Then the 3rd, the 2nd, the 1st. It teaches the drop, and it builds the strength gently.