Opposite to many opinions, the violin must be played like the guitar. Why? Because there we have 4 strings or 4 fixed notes. To learn and memorise the places of all the notes for example in first position we must use maximum checks possible. The place of the pinky is very easy to find because in first position pinky must play unison with the upper string.Example: On G string we play with pinky the note D and we verify the unison with the D string.
But first little theory concerning the intervals. One octave is the interval where the frequence is twise. If the note A4 (your A string) is 442Hz then the note A on the E string 3th finger is 884 Hz (ratio 1/2). If we divide this octave in 12 half tons and then try to divide the fifth A-E (your strings A and E-ratio 1/1.5) in 7 half tons, we will be surprised than the half tons from the fifth are bigger! Or the first condition to play in tune is...to tune correctly the instrument. Or after taking the A from a electronic tuner we can tune E string with a pure fifth because after this string will be hot and will go slightly down-the fifth will become narrow. Then we tune the D string maximum hight possible to avoid the pure fifth D-A. Then we must rechecking E string if A string remain in tune.After that we tune narrow fifth G-D strings and if needs we can verify other strings. The basic problem when we tune the violin is that all strings use the same point of support and when we change one string the other ones strings change also...
Now we can see where are other correct places for our fingers. The 3rd finger on E string must form a octave with the string A but not a pure octave but slightly stretched one! For the note C on G string the place will be exactly in front of the note G on D string. We continue with the second finger. If we have to play a minor third with the upper string (for example 2nd finger on A string-C sharp and the E string), we will try to play this C sharp maximum hight possible to have one narrow minor third. If we check a major third-there we must try to play a stretched third. Or in the same example C-E the second finger for C must be lower possible.For checking the normal place for the first finger we have two possibility-4th with the upper string or major 6th with the lower one.Where the 4th must be stretched and the major 6th also. If we use low position for our first finger then the right place will be a narrow minor 6th with the lower string-for example A-F (first finger on E string)
In general we must remember that only the unison must be pure.
The minor 3rd, the 5th and the minor 6th-narrow. The major 3rd, the 4th, the major 6th and the octave-stretched.
All that seems complicated but it is very useful to memorise these places! It will permit to stay in tune all the time(if the violin also stay in tune).
All corrections for play narrow or stretched intervals are very little ones and only your ear will confirm if the place is correct.