ושמעו לקלך AND THEY SHALL HEARKEN TO THY VOICE — as soon as you will mention to them this expression (the double use of the verb פקד as in v. 16) they will hearken to your voice, for they have long had this sign as a tradition from Jacob and Joseph that by mention of this phrase their deliverance will be brought about. Jacob said to them “And God will surely visit you”, and Joseph said to them (Genesis 50:25) “God will surely visit you”.

But Moses spoke up and said, “What if they do not believe me and do not listen to me, but say: ה׳ did not appear to you?”

(29) Then Moses and Aaron went and assembled all the elders of the Israelites.(30) Aaron repeated all the words that ה׳ had spoken to Moses, and he performed the signs in the sight of those assembled,(31) and the assembly was convinced. When they heard that ה׳ had taken note of the Israelites and that [God] had seen their plight, they bowed low in homage.

(9) But when Moses told this to the Israelites, they would not listen to Moses, their spirits crushed by cruel bondage.(10) ה׳ spoke to Moses, saying,(11) “Go and tell Pharaoh king of Egypt to let the Israelites depart from his land.”(12) But Moses appealed to ה׳, saying, “The Israelites would not listen to me; how then should Pharaoh heed me, me—who gets tongue-tied!”*who gets tongue-tied Lit. “uncircumcised of lips,” which is not a claim to a permanent condition, contra NJPS “a man of impeded speech.” Cf. Lev. 19.23; Jer. 6.10.

The people had good reason for becoming impatient at their fate because when Moses had come he had given them hope that their liberation was close at hand. This had given them a new and broader perspective on life. Now, when Pharaoh had decreed additional hardships their minds could concentrate only on how to cope with the immediate and even worse situation. The word מעבודה may therefore be understood as something comparative, i.e. the even greater bondage than had been their lot prior to Moses' coming.

AND MOSES SAID UNTO THE ETERNAL: O LORD, I AM NOT A MAN OF WORDS, NEITHER YESTERDAY NOR RECENTLY NOR SINCE THOU HAST SPOKEN UNTO THY SERVANT; FOR I AM SLOW OF SPEECH AND OF A SLOW TONGUE. “This teaches us that for an entire period of seven days, the Holy One, blessed be He, sat and urged Moses to undertake the mission. [The expressions], ‘yesterday,’ ‘recently,’ and ‘since Thou hast spoken,’ imply three days, and the three-fold word gam — [here translated ‘neither’ or ‘nor,’ but literally meaning ‘also’] — points to a similar extension of time. Thus you have six days [that have passed], and it was now the seventh day [when Moses still refused to go on his mission].” Thus the language of Rashi.
In line with the plain meaning of Scripture, the purport of the verse is as follows: [Moses said,] “For I am slow of speech from heretofore and from time past, for I have been slow of speech from my youth on and all the more now that I am old, and also now since Thou hast spoken unto Thy servant, for Thou hast not removed the defect in my speech when Thou didst command me to go to Pharaoh to speak in Thy name. How then can I go before him?” Now Moses out of his great desire not to go [on the mission] did not pray before G-d, blessed be He, that He remove his defective speech from him, but he argued: “Since You have not removed my slowness of speech from me from the time You spoke to me to undertake this mission, do not command me to go, for it is inconceivable that the Master of everything should send a man of uncircumcised lips to a king of the nations.” And since Moses did not pray [for the removal of his defect], the Holy One, blessed be He, did not desire to heal him. Instead, He said to him, I will be with thy mouth, and teach thee what thou shalt speak, meaning that “you will be able to correctly express the words which I will put in your mouth.”
And in V’eileh Shemoth Rabbah, the Rabbis said: “The Holy One, blessed be He, said to Moses: ‘Do not mind it that you are not a man of words. Have I not made the mouth of all that speak, and him that I desire I made dumb? And have I not made the deaf and the blind, and opened their eyes to see and ears to hear? Now had I wanted that you be a man of words, you would have been so. But it is my desire that you continue to be so, and when you will speak [to Pharaoh] your utterance will be correct, for I will be with thy mouth.’ This is the sense of the verse, Now therefore go, and I will be with thy mouth.”
According to this Midrash, it appears to me that the reason He did not desire to remove his defective speech from him was because a miraculous event, as told by our Rabbis, happened to Moses when he was still before Pharaoh.
The correct interpretation appears to me to be that G-d said to Moses, “Who hath made man’s mouth? or Who maketh a man dumb? … Is it not I the Eternal Who does all this? I could heal you. But now since you did not want to be healed, nor have you prayed to me about it, go and I will be with thy mouth, and I will cause you success in My mission.” It is also possible that there is a hint in the verse, And the anger of the Eternal was kindled against Moses, that He did not want to heal him, and that He sent him against his will.

בשנאת ה׳ אתנו BECAUSE THE LORD HATETH US — Really, however, He loved you, but you hated Him. A common proverb says: What is in your own mind about your friend, you imagine is what is in his mind about you (Sifrei Devarim 24:3).

There are some who have the custom before taking three steps or reciting, My God, guard my tongue from evil, based on the verse, “Guard your tongue from evil, your lips from deceitful speech.” (Ps. 34:14) Though good and evil are in the hands of the human being, one asks God to help us do good.

AND MOSES SPAKE BEFORE THE LORD, SAYING: BEHOLD, THE CHILDREN OF ISRAEL HAVE NOT HEARKENED UNTO ME, HOW THEN SHALL PHARAOH HEAR ME, WHO AM OF UNCIRCUMCISED LIPS ? How did Moses dare say this? Had not the Holy One already promised him, when he said that he was not eloquent, that He “will be with his mouth” (Ex. 4, 10-12)? Or did the Holy One not keep His promise?