Is it bad to have have a steel D string on a classical guitar? My D string is steel; does this put to much pressure on the neck of the guitar? I bought the guitar set up like this about a year ago and I have been learning on it ever since, it tunes fine.
Classical guitars are not made to withstand the pressure of steel strings, so yes it is bad to have a steel string on there. At least it is only one strong because with any too much force (more steel strings) the bridge can be pulled off the guitar and the neck warped, so definitely change the string as soon as possible.
If all 6 were steel, you need to worry. One won't make a lot of difference, more tension on the neck, but mainly on the bridge. I see similar guitars where the bridge has pulled off the body. There is also the difference in sound of that 4th string, and the fact that you have to press harder to fret it. Makes sense to change it, but if the others have been on a long time, it wouldn't hurt to change all of them.