Which of these sentences are accurate in terms of tense aspect? Tense in a conversation is important so as not to get the interlocutor puzzled, right?
1. You speak too quickly. I can't catch on to what you are saying?
2. You spoke too quickly. I can't catch on to what you have just said?
#1 means that the person is still speaking and you are having trouble understanding anything they are saying throughout the conversation because they speak too quickly.
#2 means that you didn't understand the last few words the person said, because they spoke too quickly.
Note that "catch" would be better here than "catch on".
Sorry, I didn't catch what you said.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.