Why did Hashem need to remind Moshe to carry out his instructions?
(4,21) “And Hashem said to Moshe: When you go to return to Egypt, see all the wonders which I have put in your hand and do them before Pharaoh.”
Since Hashem had already instructed Moshe to go to Egypt and had instructed him about the signs that he should perform, what new thing was Hashem telling Moshe now? And why did Hashem think that Moshe would not carry out his instructions and so He needed to warn him again?
But the explanation is that if someone sends a messenger to perform a certain task there is no need to repeatedly remind to do it, because it can be presumed that he will do it since he is going there for that specific purpose. But if someone is travelling somewhere for his own purpose and his friend asks him to do there something also for him, then he does need to remind him so that he will not forget to do what he asked him.
Now, the difference between the expression 'going' to the expression 'returning' is that if someone lives with his family in a certain place and he and all his family uproot their dwelling from there to another place, if he subsequently returns to his original place by himself without his family it would not be correct to say he is returning but rather that he is going. But if he takes all his family with him then we would say that he is returning.
Thus, initially when Hashem said to him “Come now, and I will send you to Pharaoh“ (Shemos 3:10), He did not say to him "return to Egypt" since it was Hashem's will that he should go alone without his wife and sons, and thus he was only 'going'. But afterwards, when Moshe wanted to also take his wife and sons, he said to Yisro "Let me go now and return to my brothers who are in Egypt", and it says in a later posuk that "Moshe took his wife and sons".
Therefore, initially, since it was Hashem’s will that Moshe go alone, he was going only as an emissary, and thus Hashem did not have to caution Moshe to carry out his instructions since this was his only reason for going to Egypt. But now that he was taking also his wife and sons it was clear that he was retuning to Egypt to establish his dwelling place there, and thus there was a concern lest he be negligent in carrying out his instructions. Therefore, Hashem reminded him "when you go and return to Egypt" - when you return to Egypt with your wife and sons, remember to “see the wonders which I have put in your hand and do them before Pharaoh”.