נביאך, your spokesman.

ושלח, he should tell Aaron to tell Pharaoh to let the Israelites go.

Egypt will know that I am ruler and master of the whole universe. This will include all the people who up until now had refused to acknowledge that there is such a G’d.

ויחזק לב פרעה, he claimed that Aaron had also merely performed a better kind of sorcery.

כבד, Pharaoh had hardened his heart, as I explained already. (the word is a verb, not an adjective, compare Exodus 1,8 on the word מלא). In intransitive words applying to the body of a person, such as vayishman, he waxed fat, (Deut. 32,9) or vayichbad, he remained heavy, (Exodus 9,7) and several similar examples, constructed by a future tense with the prefix ו, the ordinary past tense would be kaved, or shamen.

הנה יוצא המימה, as do most ministers, in order to take their morning stroll. Sometimes they go for a short gallop on their horse.

'בזאת תדע כי אני ה, to teach you who have said: “I do not know any god by the name of Hashem.” (5,2)

ונלאו, I explained the origin of this word already on Genesis 19,11. It is a reinforced version of saying “they could not.” It is an expression of being powerless.

לעיני פרעה, this was a big boost for Moses’ stature, that Aaron, merely his messenger, was able to accomplish this and use his staff on a regular basis.

מתה, the accent is on the first syllable as the verb is in the past tense. When the same verb occurs in the present tense/future tense, as in Genesis 30,1 the accent is on the last syllable. In Genesis 48,7 when Yaakov describes Rachel having died on him, the accent is also on the first syllable.

גם לזאת, seeing he had already made himself obstinate, vayechezak, (verse 13) at this time also, he relied on his own resources of self confidence to ignore what he had seen. However, when it came to the plague of frogs Pharaoh already began to waver and external stimuli in stiffening his attitude were required. Hence we read there (8,11) vayachbed Paroh et libbo, that Pharaoh had to draw on external stimuli to maintain his obstinate pose. The same was true with the fourth plague, that of the wild beasts invading civilised urban regions. (8,28)

שבעת ימים, this was the duration of the plague that struck the river.

ויאמר ה' אל משה בא אל פרעה, Moses warned Pharaoh twice before a plague materialised, whereas before the third plague he did not give any warning. This remained the pattern throughout all the plagues which occurred in groups of three. Before the plague of blood and frogs Pharaoh was warned, not so before the vermin. Before the plague of free roaming beasts and pestilence Moses extended a warning, whereas he did not warn Pharaoh of the plague of boils and carbuncles. Pharaoh was warned both before the hail and the locust, whereas the plague of darkness occurred without prior warning.

נוגף, another word for makkeh, “strike, smite.”