catch on in English:

I understand.

I don't understand and I'm not used to not understanding. Please explain it once more.

Where is there a drugstore where they understand English?

It is easier to write a bad poem than to understand a good one.

The more I think about it, the less I understand it.

What I don't understand is why Tom didn't stick around and help us.

I understand the general approach, but I'm afraid that I'm getting bogged down in the details.

Any universe simple enough to be understood is too simple to produce a mind able to understand it.

A person with average intelligence would understand that.

I could understand the first few sentences, but the rest of his speech was all Greek to me.

The most essential thing in the world to any individual is to understand himself.

It requires wisdom to understand wisdom: the music is nothing if the audience is deaf.

You may not learn to speak as well as a native speaker, but you should be able to speak well enough that native speakers will understand what you have to say.

How much of what I've written here do you understand?

I understand you are going to spend your vacation in New Zealand.