I recently bought a guitar from a friend and it has nylon strings. He said it originally came with steel strings, but for some reason he replaced them with nylon. I'd like to know if it will bring any problems to my guitar if I replace the nylon strings with steel again, since the guitar is already used to the nylon strings tension.
Hold on here. If this particular make and model of guitar was indeed originally designed for steel strings, but your friend put nylon strings on it, then it would indeed be okay to put steel strings on it instead.
However, we need more information. What is the exact make and model of the guitar? What modifications have been made to the guitar to accommodate nylon strings? Were they ball-end "folk"-style nylon strings inserted into the original steel-string-style bridge and bridge plate? Or was some other arrangement made?
Please post the exact make and model of the guitar. Some detailed photographs of the headstock, nut, and bridge would be helpful.
It would be a good idea to take the guitar to a reputable guitar repair technician, have them inspect the guitar, and ask them about the feasibility of switching to steel strings.
If the guitar technician says this is OK, you would expect to need to pay for modifications and adjustments to be made to the guitar so that it is properly set up for the new strings and so that it plays in tune. Steel strings have more than twice as much tension as nylon strings, and they are different thicknesses and diameters, so modifications or adjustments will probably need to be made to the nut, the truss rod (I hope this guitar has a truss rod) and possibly to the bridge saddle and bridge.
You could try to do these things yourself, but in my opinion, paying money to a professional would result in a guitar that is more playable.
Guitars designed for steel strings are made completely different from guitar's designed for nylon strings. Putting steel strings on a guitar made for nylon strings could ruin the guitar. Putting nylon strings on a guitar made for steel strings is not a good idea either.
A picture of the guitar might help me determine which type of guitar you have. But there are some clues you can look for.
Guitars designed for steel strings will almost always have a truss rod to compensate for the very high tension that steel strings exert on the neck. If your guitar does NOT have a truss rod, you should not put steel strings on it.
A truss rod is adjustable with a special wrench (often an allen wrench) and exerts counter tension on the neck to prevent the string tension from warping it. Truss rods run through a channel in the guitar's neck and are accessible either through the sound hole or by removing a small plate on the headstock directly behind the nut.
Some nylon string guitars also have truss rods, so the existence of a truss rod does not automatically mean the guitar is designed for steel strings. But the absence of a truss rod definitely means you don't want to put steel strings on the guitar!
If you put nylon strings on a guitar with a truss rod, you would want to loosen the truss rod so that it does not warp the neck by exerting tension that is not adequately counter balanced by the string tension.
Another significant difference between guitars meant for nylon vs. steel strings would be that the string slots in the nut would be much wider for nylon strings, especially the treble strings, given their larger diameter.
The bridge on a guitar designed for steel strings will usually be reinforced more to handle the extra tension. Putting steel strings on a guitar with inadequate bridge reinforcement could result in the bridge pulling loose from the top of the guitar.
Most steel string guitars have bridge pins and the ball end steel strings are inserted through the bridge itself into the body and held to a bridge plate on the inside of the guitar by the bridge pins. This transfers most of the string tension to the bridge plate on the inside of the guitar, allowing the bridge to withstand the greater tension of the steel strings. Nylon string guitars are often strung by tying the nylon strings around the bridge as opposed to through the bridge into the body.
The tuning mechanism and gears are more robust in guitars designed for steel strings as well.
Bottom line - if your guitar was built for and designed for steel strings, you should change the strings back to steel and adjust the truss rod for the correct amount of counter tension and desired amount of relief. If the guitar was designed for nylon strings or does not have a truss rod, you should not put steel strings on it.
Again - several pictures from different angles including close ups of the headstock and bridge might help us determine which kind of guitar you have.