My prof always told me to keep it simple.

If you use complicated language, there are mainly three reasons:

- You don't understand what you are talking about - You don't want to be understood - You want to sound smart

I believe that an explanation should go straight to the core. If you truly understood something, it should be of no problem to explain it to your in-laws. (Ironically, this sentence is condescending..)

Overall, I don't think I ever thought that someone is condescending when they went to honestly explain something to me.

But it depends on the circumstances. Making a mistake and getting someone to explode "why don't you understand this simple thing?!" and then having them explain it to you is a miserable experience, for everyone involved.