I couldn't argue with him so I agreed.
Why weren't you able to argue with him?
I'm afraid he might be right
I understand your feelings. It's good to keep the peace in a relationship. It is okay to give in if you think he might be right, but in the future you shouldn't give in so easily and defend your points.
Don't you know, arguing is a waste of my time.
You are entitled to think that way and stay peaceful, and you might have more productive things to do than argue. But I believe you shouldn't keep doing this, because if you always let him get his way he might just assume that from now on.