Student or Learner
I came across this question just now. Here, I'm describing what I did a while ago. I know I could say they couldn't understand to indicate they didn't have the ability then. Suppose I went to a place where senior citizens spend the rest of their lives and the fact they don't understand my language extends from the past into future. Is it correct to use the sentence in the title?
It sounds fine to me.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.