Thanks guys. I had asked the same question here (I'm living in Czech Republic) and people said the same, no difference. But just now I understand the difference in terms of formality.

"Jen" is used in formal language whereas "Jenom" in informal, but there's not any difference between these word. The only problem may occur in phrases ex. "Jen počkej!" (You give someone clear that you will revenge him) -> In this phrase is always used "Jen", you can use "Jenom" but it is very unusual. But as I said, don't worry about these words, you can use both of them without changing the meaning :)

"Co se týče oblečení, koupím si něco nového, jenom když je něco skutečně nepoužitelné." <- In this sentence you can use them both. I don't see a reason why someone would correct it. There is no difference in meaning, unless someone is pedantic and wants to change at least something.

Ok. I have one recent sentence. I wrote:" Co se týče oblečení, koupím si něco nového, jenom, když je něco je skutečně nepoužitelné" and someone changed the "jenom" to "jen". As this has already happened before, I was wondering about possible differences. I normally practise writing on lang-8 but when something isn't clear, I don't always get an answer.

Hello Daniela! Could you, please, write some sentences where the two words are (in your opinion) not interchangeable? I see the only difference (and it is a really tiny difference) in level of formality. For example in formal essays I'd tend to use "jen" and on the other hand in spoken language I'd prefer "jenom". There are also some fixed expressions, where one is preferred over the other, so maybe you mean those? (Jen tak dál. - To bylo jen tak tak. - etc. ...)