Question: How can we differentiate between 'Smaller' and 'Greater sins'?
Answer: Actually the source of judging the Smaller and Greater sins are the verses of the Holy Qur’an from whom is the following: “If you shun the great sins which you are forbidden, We will do away with you small sins…”[1]
Now let us see what is the scale of determining whether a sin is small or big
Our scholars say that every sin, be it small or big is a great sin in itself.
But the scale of finding whether a sin is small or big is not that it should be inspected with relation to the Divine realm. Because from the aspect of such investigation it is all 'great' even though this division of sins is done to compare between themselves. According to which there are two kinds of sins, 'Greater' and 'Smaller.'
Now we should see what is the scale of differentiating the two? There are many techniques to discriminate the two of them and the most famous of them according to scholars is that every sin regarding which the Holy Qur’an has promised punishment is greater sin. For example, murder. Regarding which the Holy Qur’an says: “And whoever kills a believer intentionally, his punishment is hell; he shall abide in it…”[2]
Some scholars have added another condition also that greater sin is that for which punishment is promised or it is emphatically prohibited. Because there are many sins which are not promised punishment but they are emphatically and repeatedly prohibited.
For example if we consider that only the following sentence has arrived for usury that those who take usury have made a declaration of war against Allah. Then this alone is a sign that usury is a greater sin. And if in other ayats and traditions if the usurers had not been promised hell, even its emphatic and repeated prohibited shows that it is a greater sin.
From this aspect the magnitude of sins is not relative. Their limits are different. It can never be that a sin is smaller as well as greater. Because if it is promised punishment of if its performance is emphatically prohibited, it would be a greater sin, otherwise it is smaller.

Are Greater And Smaller Relative Terms?
Question: Some people say that the greater and smaller sins are comparable (ratio) that is it is possible a sin is greater but in comparison to another sin it would be considered small. For example, theft is a greater sin but in comparison to intentional murder it is considered small. From the aspect of their comparability all the sins are eligible for forgiveness and remission. Because the Holy Qur’an says: “If you shun the greater sins we shall do away with your small sins.”
Thus every sin that is shunned keeping in view a greater one will be forgiven but there is no sin greater than polytheism that if one shuns it one could be forgiven his polytheism.
On the basis of this one who has not made a partner to Allah will have all his in forgiven.
Answer: In the reply to the previous the reply to this question has also become clear. Because as mentioned above the classification of sins into small and great is an absolute clarification which is divided into two on the basis that some are those which are promised divine punishment or prohibited emphatically and some are not.
They have not been classified due to their magnitude or on the basis of conjecture. And the captioned ayat discusses this point and the conclusion derived from it is that if a Muslim refrains from one class of sins that is the greater sins, the Almighty Allah will forgive their sins of the second class, that are called Sayyat or Saghaer - that is smaller.
And there are two separate limits of the two different classes of sins and none of them can overlap. Therefore, shunning the greater sins cannot cause the other greater sins to be forgiven. It can only help in forgiveness of smaller sins, (In the background we should also remember that repeating smaller sins make them into greater sins).

How Many Greater Sins Are There?
Question: How many greater sins are there?
Answer: There are different methods to recognize the greater sin and we indicate towards two of these methods.
1. Every action the doers of which have been promised by Allah punishment and chastisement in the Holy Qur’an. (Whether, it is clearly stated or indicated or it may be emphatic prohibited again and again). It is a greater sin.
2. Through the correct and authentic traditions that have reached us from the great leaders of Islam and in which the greater sins are clearly enumerated. For example the letter Imam Ali Ibn Musa ar-Rida (a.s.) had written to Mamoon the Abbaside Caliph. In the letter he has mentioned the number of greater sins. And in the same way is the tradition narrated by Amash from Imam Ja'far as-Sadiq (a.s.). There is mention of many greater sins in this tradition. And the same is the condition of other traditions.
Now we shall describe a greater part of the greater sins which mentioned in ayats or traditions and pray to the Almighty Allah that He may give us the Taufeeq to shun all these sins.
1. Murder 2. Adultery 3. Wine 4. Leaving Prayer 5. Usurping the property of orphans
6. Fleeing the battlefield 7. Interest taking 8. Gambling 9. Oppression 10. Sodomy
11. Helping the oppressors 12. Supporting the oppressor 13. Denying the rights of people 14. Fighting the saints of Allah 15. Lying 16. Betrayal of trust. 17. Hurting the Parents
18. Accusing pure people falsely 19. Backbiting 20. Careless of the Anger and Punishment of Allah 21. Carelessness about performing the Hajj 22. Squandering and over spending 23. Pork 24. Blood 25. Eating dead meat 26. Eat the flesh of animal which is slaughtered without reciting the name of Allah 27. Despair from mercy of Allah 28. Weighing less. 29. Repeating smaller sins 30. Breaking off relations 31. Non payment of Zakat 32. Breaking of covenant 33. Concealing Testimony 34. False swearing.
The above-mentioned sins are a major portion of greater sins though according to the view of many scholars they are not limited to these.

Notes:
[1] Surah Nisa 4:31
[2] Surah Nisa 4:93