Genesis 4:7

Its explanation accords with that of the Targum.

At the opening of your grave, your sin will be waiting for you...

The sin [desires you]; this is the yetzer hara, which forever desires and yearns to trip you up.

If you wish, you may overcome it.

If thou doest thy work well, will not thy guilt be forgiven thee? But if thou doest not thy work well in this world, thy sin is retained unto the day of the great judgment, and at the doors of thy heart lieth thy sin. And into thy hand have I delivered the power over evil passion, and unto thee shall be the inclination thereof, that thou mayest have authority over it to become righteous, or to sin.

IS IT NOT THUS, IF THOU MENDEST ‘SE’EITH’. In the opinion of the commentators, this means there is a “lifting” or forgiveness of your sin. And in the opinion of Rabbi Abraham ibn Ezra it means a “lifting” of your face in contrast to [the question G-d asked of Cain]: Why is thy face fallen? For he who is ashamed presses his face downward. Similarly, it is said, And the light of my countenance they cast not down, whereas one who honors him is as if he raises his face upward. This is the sense of the verses: Perhaps he will lift my face; Do not lift the face of the poor.
In my opinion the verse means: “If you will mend your ways you will have your rightful superiority in se’eith (dignity) over your brother since you are the firstborn.” And this is the meaning of [G-d’s question to him]: Why art thou wroth? For by virtue of his feeling ashamed before his brother, his face fell, and because of his jealousy of him he killed him, and now the Eternal told him: Why art thou wroth regarding your brother, and why is thy face fallen on account of him? Is it not thus! If thou mendest, you will have superiority in dignity over your brother, and if thou dost not mend, evil will come upon you not only because of him [your brother], for at the door of your house your sin lurks causing you to stumble in all your endeavors.

AND UNTO THEE IS ITS LONGING, for your sin longs to cleave to you at all times. Nevertheless thou mayest rule over it if you so desire, for you may mend your ways and remove it from upon you. Thus He taught him [Cain] concerning repentance, that it lies within his power to return anytime he desires and He will forgive him.

הלא אם תיטיב, if only you improve yourself you will also be welcome in My eyes.

שאת, every lofty level of greatness is lying in wait for you, ready to become yours,

ואם לא תיטיב לפתח חטאת רובץ, however, sin is also lying in wait for you if you will add further intentional sin to the already performed unintentional sin. This is the way of the evil urge, that once you give into it, it will forever be ready to tempt you again.

כי אליך תשוקתו, “for it longs for you due to its very nature. It too is anxious to satisfy its desires, its aspirations every day.”

ואתה תמשול בו, it is within your power to overcome the power of the evil urge thanks to the צלם אלוקים, the divine image with which you have been provided at birth. This conforms to our sages in Sukkah 52 who have taught that were it not for this צלם אלוקים, which represents G’d’s assistance against the evil urge, there would be no way man could cope with temptation. This is what David meant in Psalms 37,33 ה' לא יעזבנו בידו ולא ירשיענו בהשפטו, “the Lord will not abandon him to his power; He will not let him be condemned in judgment.”

הלא אם תיטיב שאת, when your heart will be good and your deeds will be of benefit for you. The meaning of such terms as כפרה, סליחה, is like נושא עון in Exodus 34,7. The scholar Rabbi Avraham Ibn Ezra explains the concept שאת mentioned in our verse as derived from נשיאה the lifting of something, the opposite of ויפלו פניו, Kayin’s face having “fallen.” G’d told Kayin that if he were to become good, mend his ways, then his face would be “lifted,” he could hold his head high. G’d Himself would raise his face for him. In Bereshit Rabbah 22,6 the words אם תיטיב שאת are understood as a blessing, whereas the words ואם לא תיטיב are understood as a curse, quoting Leviticus 20,19את עונם ישאו. Another approach, also taken from the Midrash: “if you will improve your ways I will forgive all your previous sins, if not the very sin you were guilty of will weigh even more heavily in the scale.” (seeing you have spurned My offer of forgiveness). Rabbi Berechyah, quoting Rabbi Shimon bar Ami, explained Psalms 32,1 אשרי נשוי פשע כסוי חטאה to mean: “hail the man who towers above (is higher than) sin,” as opposed to the sin which towers above man.”

ואם לא תיטיב לפתח חטאת רובץ, as if the Torah had written not in general terms לפתח, “at the entrance,” but לפתחך, “at your entrance.” The word רובץ, crouches, lies in wait, is in the infinitive to underline that the temptation to commit a sin is ever present for such people. Sin is a constant peril, an obstacle waiting to trip you up. It awaits you both coming and going. The apparent contradiction between the masculine mode רובץ, and the feminine mode for sin, חטאת instead of חטא, is a reminder of the evil urge, the נחש, the serpent, masculine, the force causing sin, being very powerful.

ואליך תשוקתו, its desire is centered on you; it is constantly engaged in trying to seduce you by making you stumble.

תשוקתו, a word meaning desire, greed, lust. The word also occurs in this context (more or less) in Song of Songs 7,11 ועלי תשוקתו, “His longing is for me.”