Can you put nylon strings on a steel string acoustic guitar?

Absolutely not. Steel string guitars are set up and braced for the tension of steel strings, which is much more than nylon strings. Throw the nylon strings away and get a set of steel strings. One practical matter is that steel string guitars have a pin bridge and require strings with a ball end, which nylon strings do not have. You should not leave any strings on for a year - change them much more frequently than that. You can find inexpensive strings for as low as $3 - $4 a pack.

It won't sound the best but it also won't damage the guitar. If you don't mind the tone, it's ok. A steel string guitar has a stronger neck then a nylong string guitar, so you can put nylon on a steel guitar but should not put steel strings on a nylon guitar. I'd guess the total tension across 6 nylon strings is about half the total tension across steel strings, so the nylon strings put far less force on the guitar. If you play mostly chords, it will work fine. If you fingerpick then it will be a little bit crowded, because the steel string guitar usually has a narrower neck. I disagree with the first response above. Why would nylon strings become any looser on a steel string guitar? The string control is a function of the tuning knobs, it has nothing to do with the neck or the body. Those give the tone, but don't control the strings. As long as the tuning pegs don't slip, the nylon strings will react in stretched position exactly the same as if they were on a nylon string guitar... or even the same if they were stretched on a fence post, for that matter.