This verse is difficult to interpret, as can be seen by comparing different translations. The questions in verse 6 show that the LORD does not approve of Cain’s reaction, and so he goes on to advise Cain what he should do.
If you do well means “If you behave, conduct yourself in the right way.” Will you not be accepted? is a rhetorical question in Revised Standard Version that expresses the consequence of the condition; that is, “If you do what is right, you will certainly be accepted,” or “I will surely accept you,” “I will not at all reject you.” Be accepted is literally “[there will be] lifting up.” It is possible that the expression “lifting up” in Hebrew refers to the countenance or “face” that has fallen in verses 5 and 6, and so the meaning may be “lifting up the face,” or as Good News Translation says, “you would be smiling.” New English Bible has “accepted” in the text and “You hold your head up” in the footnote. New Jerusalem Bible translates “You ought to hold your head high,” Bible en français courant “You will get the upper hand again.”
The first part of the verse appears to give Cain a warning that it is up to him to mend his ways. The second part seems to give the alternative of what will happen if he fails to correct his attitude. Good News Translation translates the first “if” clause as past time: “If you had done….” The Hebrew form of “do well” is in the imperfect, which is more commonly translated here as present: “If you do well” (Revised Standard Version, New English Bible and others), “If you do what is right” (New International Version). In many languages the passive form will you not be accepted? must be changed to the active: “Will I not accept you?” “I will not reject you,” or “I will accept you.”
The second part of the verse begins with the alternative if you do not do well, and the reader expects a word to follow that balances with “lifting up” in the first part. However, no such word follows in Hebrew. Instead there is the figurative expression sin is couching at the door. The meaning of this expression is not entirely clear. It appears that sin is here pictured as an animal stretched out in rest at the entrance of its den. Although the Hebrew verb translated couching refers mainly to an animal lying at rest, sin is not passively resting, because its desire is for Cain. Accordingly sin is pictured as the animal waiting for its victim. Couching is an archaic English word referring to an animal lying at rest or in concealment ready to spring. It is in the latter sense that Good News Translation and others use “crouching.”
In some languages it will be necessary to adjust sin is couching to say “sin is like a wild animal crouching at your door.” The image of sin lying in wait for its victim suggests that sin is hiding, and consequently it may be necessary to say “crouching behind your door.” Biblia Dios Habla Hoy avoids the image of sin crouching at the door and says “sin is waiting”; it then combines the following clause, its desire is for you, to say “sin is waiting for the moment to dominate you.” Because we are translating a figurative expression, we are forced to ask what the door represents. The image may refer to the entrance to a dwelling, but it may equally well be taken as a reference to the heart, mind, thoughts of Cain. In either event it is possible that in some languages it will be necessary to shift to a simile and say, for example, “Sin is like an animal lying in wait for you at your door,” “… lying in wait at your heart’s door,” or “… lying in wait to catch you.” One translation that retains the picture of an animal in this verse says “… your bad behavior will be like a wild animal waiting close to the door ready to bite [or, eat] you.” Another translation that avoids the picture has “… if you do what is wrong, then sin is ready to destroy you.”
Its desire is for you: desire translates the same word used in 3.16, speaking of Eve’s longing for her husband. The sense is that “sin wants to be your master” or, as in Good News Translation, “It wants to rule you.” Its desire refers to the desire that sin has to master Cain (like an animal in ambush). If the translation of the preceding clause is “sin is like an animal…,” its will refer to the “sin … animal.” It is also possible to translate desire … you as an extension of couching at the door; for example, “Sin is like an animal crouching behind your door wanting to dominate you.”
But you must master it: that is, “you must overcome sin,” “you must rule over it,” “you must not let sin rule you.” In some languages sin cannot be the object of such verbs as “overcome” and “rule.” Furthermore sin is sometimes not expressed as an abstract noun but only as a verb phrase. Therefore some adjustments may be required. For example, we may say negatively, “You must not let the evil things you do be a chief for you,” “You must say ‘No’ to the bad things you do,” “You must not obey your desire to do evil things,” or “You must command yourself so that you do not do what is evil.”
Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .