לאמר, this refers to verse 4 in which Moses referred to the פנים אל פנים “face to face” encounter when G’d addressed the Jewish people directly at the revelation on Mount Sinai commencing the Decalogue with the words “I am the Lord your G’d, etc.”

על פני. I believe that G’d’s point is that even if you believe in Him absolutely and exclusively, you must not make any tangible symbol, anything that you would use as a charm, such as Bileam who did believe in G’d but used charms to help him establish communication with G’d, much as mediums use techniques involving bodily contact. Lavan even called such tangible symbols as his teraphim: “e-lohay,” (Genesis 31,30)

The prohibition in Exodus 20,22 not to make “gods of silver or gold,” is not intended to suggest that Jews would consider such man made objects as being divine, but they are a form of establishing indirect rather than direct contact with G’d, contact without resorting to anything physical tangible. G’d made the same point repeatedly, such as in Exodus 22,19.

שמור את יום השבת, I have already explained in Exodus 20 why this time the expression שמור is used instead of the expression זכור which the Torah used there.

כאשר צוך ה' אלוקיך, a reference to G’d having explained the rationale for Sabbath observance in the paragraph dealing with it both in Parshat Yitro (Exodus 20,10) as well as in Parshat Ki Tissa, and Parshat Vayakhel. The expression כאשר צוך used in connection with a positive commandment means that failure to observe it is simultaneously the violation of a negative commandment. We do not find individual negative commandments accompanied by a warning: “which the Lord your G’d has commanded you not to do.”

ולא יסף, according to the plain meaning of the text there was never again such an overpowering sound in the history of mankind. Whenever the word יסף appears it represents an addition of something. (compare Deut. 5,21, Psalms 73,19, and Genesis 18,23, the expression האף תספה, where the latter word is from the same root as יסף.)

'ואתה עמוד פה עמדי ואדברה אליך ...ועשו בארץ וגוthis is a reference to when Moses had said in 4,14 “G’d commanded me at that time to teach you to perform the commandments applicable in the Holy Land.”