Does anyone have a solution to the problem of (1) fretting a string and touching the adjacent string with the fretting finger and (2) having the fingernail touch the fret board while fretting a string preventing a good sound? The two problems are related in that problem (2) can be solved by placing the fingertip on the string close to the nail, but problem (1) becomes more pronounced because the fleshy part of the finger extends to toward the adjacent string and often touches it. If the fretting fingertip is centered on the string to address problem (1) the fleshy part of the fingertip there is soft and does not apply sufficient pressure to fret the string properly creating problem (2).
Takato: With continued practice you will develop calluses on the tips of your fingers and you will develop independent finger strength. In my opinion you should cut or file your left hand nails and also work on curving your fingers so they do not touch adjacent strings.
Hope this helps.
I would say if your nail is touching the fret board it's too long, but I'm sure you've thought of that. Does your guitar have very low frets? The average fret height (fret board to the top of the fret) is abut 1 mm. If the guitar is older and has had the fret board "dressed" to compensate for fret wear it could be lower. In that case new frets might be the answer. If you were to consider new frets you could ask the luthier to put in higher frets. While 1 mm is pretty standard there are frets available at 1.09, 1.14, 1.19, 1.27, 1.39, 1.44 and 1.47 so that is worth a try. The other thing is the string spacing. Again the average neck width at the nut is 52 mm but 53 and 54 are fairly common. The string spacing on a 52 mm nut is typically 42 or 43 mm. A luthier could make you a nut to give a bit wider spacing, say 44 or 45. That could become a problem as the 42 mm string spacing gives 5 mm from the low or high e to the edge of the fret board. Making this smaller and there is a danger of pulling the string off the board when playing. There is more danger of the string coming off on the treble side than on the bass side so what can be done is move the high e out by .5 mm and the low e by 1 mm. That will widen the spacing by 1.5 mm which might be enough to fix your adjacent string problem and it's relatively inexpensive to get a new nut made and it's not permanent, you can always switch back to the original nut, or if the new nut is good and you don't have any issue with the distance to the edge of the board you could try another nut with a bit wider spacing. I have seen guitars where there was material added to the width of the fingerboard to give more room. As long as it's not too much it shouldn't create any playing problems but it's a job for a luthier and you would be permanently altering the instrument but if you have large fingers and that's going to prevent you from playing it's worth it.
I often have a similar problem, especially with my 3rd fretting finger (ring finger). It remains too straight and doesn't bend enough on the 2nd joint, thereby muting the immediate string underneath. I have this problem in the first three frets, especially while playing arpeggios on chords like B7 or C7 where I have to place all my fingers on multiple frets and strings. Often, I end up curving my wrists too much, which I realise is bad technique. Haven't been able to correct it yet.
I am sure you know also, that sometimes you can push, or pull a string out of the way. I also find the closely trimmed nails on the left hand and careful shaping of the 1,2,3 fingers on the RH make a significant difference. In my case, sometimes holding a LH finger perpendicular to the fret too long sometimes leads to a cramp (trigger finger); so a "commando action" - just hold and relax as soon as the note allows.