How do I repond to claims that you don't understand?

Because it is hard to refute arguments properly when I do not understand it.

There are three ways this can happen.
1. You don't understand the concepts in the argument
2. Your opponent failed to use clear language
3. Your opponent deliberately do it.

In a online debate, I had this argument
My Opponent: No half evolved animals have ever been found and never will be. If scientists couldn't find any in the last 140 years, how will we find any in the future. We are a lot more advanced than we were over one hundred years ago. With all this technology we still can't find any evidence. (this is a Appeal to ignorance)
Me: By saying their is half-evolved animals, you are implying there is fully animals (Which is false since evolution is an continuous process).
My Opponent: There can be half-evolved animals even though it supposedly never stops. Time never stops but we still can measure it. With time we get one number and another number after that and we can measure the time within those two numbers. Why can't scientist get a monkey and a human and find the half-evolved animals? Scientist can do that with any animal.

I did not understand my Opponents rebuttal to my rebuttal.

The rest of the debate if you want to read it (in hindsight there are many things that I could done better)
http://www.debate.org/debates/Evolution-is-Defensible./2/

"No half evolved animals have ever been found and never will be" and "There can be half-evolved animals even though it supposedly never stops. " seem to contradict each other. You can simply point out this blatant contradiction to your opponent.

You should ask for further information or clarification if you don't understand a word or claim. When it comes to ambiguous words, make clear to your opponent that you want to grasp the intended meaning of his terms. Many times, it is useful to restate the claim or argument so that your opponent confirms you have a good understand of what he/she is claiming or arguing.

Don't respond, listen.