这就 can mean a whole bunch of things with different attitudes depending what you are talking about. 这就完 expresses the idea of something which doesnt take a lot of time to finish. Imagine this situation, you are waiting for your gal to finish her make-up, and she is about done, you go COME ON! and she replies, 这就完, meaning 1 sec. just a moment. when 就 is used in a short sentence, typically in an "intransitive sentence" if i can put it that way, it usually means that the time it is going to take in order to get from one action/state to the next one is fairly short. Like 这就完 from putting makeup on to the state of done doing makeup is short. If you are reading newspaper and your girlfriend asks you to wash up dishes, you can say 这就去, 1 sec, in a moment. But you'd better be moving very soon, otherwise be prepared for a funny discussion. However 就 is also frequently used when you want to emphasize the promptness of your statement, action or decision based on the result of something else. For example： 你不去， 我就不去。 I wont go because you wouldnt go 既然你没车，那我就不去了吧。I wont go because you dont have a car. 听见你喊，我就跑过来了。I ran over to you as soon as i heard you screamed. hope it helps, i probably didnt cover all the scenarios where you can use 就 but hopefully this little explanation teaser will get you warmed up. :-)

Original text: I've seen sentences start with "这就"... Such as 这就完。 Which should mean "I'm through/done." But it does make sense when 这(This) 就(just) 完 (finish/done). Does not look like the sentence meaning I provided. You are partially right, 这就完 Means exactly "I'm through/done." However your analysis is not exactly what it says. 这 here means present time, Just as the speaker is speaking, 就 here means right now or immediately. So here is the correct one : 这（as I am speaking）就 (right now, right away) 完 (done, through) 这就来 I am coming. 这就走 I am leaving 这就到 I am arriving 这就下来 I am coming down 这就吃完 I am almost done eating.

like Frank said 这就来 I am coming. 这就走 I am leaving 这就到 I am arriving 这就下来 I am coming down 这就吃完 I am almost done eating 这 means this moment here, 就 means 就要，be going to 这就， be going to + verb.+ in a minute. It also has some other meaning and usage. It depends on what 这 and 就 mean in certain situation. We can discuss them later.

就 means tend to so 就+verb means be going to do

it means you will do what you said very soon or just few minutes after..e.g.这就完：I am finishing!这就走：I will go soonafter!