If you are talking about using "on" as a location word, such as "on the ground" or "on the table", then most of the time you will use the word "en".
For example:
on the table = en la mesa
on the ground = en el suelo
"en" is pronounced similarly to the English letter "n"
In other cases, you will want to talk about the lights being "on" or the stereo being "on". In most cases, the word is "encendido" or "prendido". Note that "prendido" is used more in Latin America and "encendido" in Spain, but both words should be understood.
For example:
las luces están encendidas = the lights are on
el horno está prendido = the oven is on
Note that is the noun is feminine, the "o" at the end of the word changes to "a". If it is plural, you add an "s". That is why "encendido" becomes "encendidas" in the example above.
While the rules above will handle most cases, keep in mind that there are a dozen other cases that don't fit in these two rules. It is best to check a good dictionary. Click on the link in the resources section of this article to find out more about available dictionaries.
