Hello there! For question one, both "I got it" and "I get it" can be used as a response with a present tense meaning: I understand. With "I got it," although it looks past tense, it is still probably being thought of as present: I (have) got it now. It could also be thought of as past tense, depending on the situation: if someone explained something fifteen minutes ago, you could say "I got it" with a past tense meaning. If someone is explaining something now, you could also say "I got it" meaning "I have got it now." "I already got it" would be more clearly in the past. So to answer your question simply: Yes. For question two, you cannot just say "agree" alone. You must say either "I agree" or "Agreed." I don't think it would be useful to debate whether we should think of "agreed" as an adjective here. It is more that we have left out something. Maybe that something is "We are agreed," in which case it's fine to think of it as an adjective. But we could think of it as "We have agreed," in which case it's just a regular past participle. I don't see why it matters. I don't think there's any important difference between saying "I agree" and "Agreed." Hope that helps! These are good questions. Post more!

There might be a slight difference between "I got it" and "I get it." Both are correct and natural. "I got it" carries the idea that the other person can stop explaining, it's clear now. It is a suggestion to end the dialog, and perhaps move on to a new topic. "I get it" is less clear. It suggests that you have only just begun to understand and want to continue discussing the topic. "The best route is to take Route 123 and get off at exit 45." "I got it, thanks." "The best route is to take Route 123 and get off at exit 45." "Why not route 67?" "There's construction going on and it can clog up suddenly without warning." "I get it, you're saying Route 123 is more predictable even though it's longer?" "Yes, that's right."