What not to do with the component (Qt) under the LGPL (v2.1)

I want to use in your Qt application, I don't want to distribute source code.
Maybe later I will want to sell it, that is the app in the future commercial.

Tell me, where is the boundary of what is permitted LGPL? What I can't do not to get under (L)GPL violation?

Until I realized that I can use dynamic linking and don't modify the Qt code, that I should be enough. Is this true?

Thank you