I agree with Christa. Use "so on and so forth" any time you have a list of obvious contents to your audience. For example I will name the alphabet... "A B C D ... so on and so forth". Or I will talk about colors... all the color pigments - green, red, blue, yellow, ... so on and so forth - combine to make black. But don't use it too much and try not to use it if you are trying to be very specific/detailed, or your audience is not certain to know what you may be leaving out in your sentence. Nick

"So on and so forth" is usually used like "etc." Example: "I love all types of fruit smoothies. I like banana smoothies, mango smoothies, strawberry smoothies, and so on and so forth."

You can use "so on" anywhere you would use "etc.". For example, "colors are red, blue, yellow, and so on." "so forth" is almost never used by itself, but would be used with "so on", and to mean the same thing. For example, "colors are red, blue, yellow, and so on and so forth." It's an older and more formal phrase.

Just to add to what the others have said.. 'And so on' means 'etc' - it's telling you that what you have just mentioned are just examples - it's saving you the trouble of mentioning more. You can also say 'And so on and so forth' - it means the same but is less common. In fact, in natural informal speech we don't use either of these expressions much - we'd be more likely to say eg 'and colours like that' or 'and that kind of thing'.