Allah says in the Qur'an:
“If you avoid great sins which you are forbidden, We will expiate from you your (small) sins and cause you to enter an honorable (place of) entering. (Surah 4, Verse 31).
Therefore, a mukallaf must know which sins are great, so that he/ she desists from them. In this way, Allah, in His mercy and according to His promise, will forgive all his minor sins and mistakes.
The following list of the great sins is taken from Minhajus Saleheen (of Ayatullah Sayyid Abul Qasim al-Khoui, Dama Zilluhu):
1. To believe that Allah has any partner or colleague in His Divinity; to worship someone besides Allah.
2. To lose hope in the mercy of Allah; to believe that Allah would not save him on the day of judgement.
3. To have no fear of Allah; to think that Allah would not punish him for his sins.
4. To misbehave with parents; disobeying them or injuring their feeling.
5. To kill someone unjustly, i.e. without permission of sharia.
6. To slander a married woman, accusing her of adultery.
7. To eat an orphan's money or property unlawfully and unjustly.
8. To flee from jihad (religious war).
9. To use or eat a thing or money obtained as interest.
10. Fornication and/or adultery.
11. Homosexuality (whether between males or females).
12. Magic, witchcraft.
13. To swear falsely by name of Allah.
14. To neglect prayer (Salat) or any other wajib thing.
15. To withhold zakat.
16. To give false evidence.
17. To hide (i.e. not to give) true evidence.
18. To drink intoxicant, liquor.
19. Breach of promise.
20. To misbehave with blood-relatives; not doing good to them.
21. To migrate to a place where there might be a danger to religion or to its observance - i.e. where one would not be able to follow the rules of sharia without hindrance.
22. Theft or robbery.
23. Rejection or denial of what Allah has revealed to the Prophet (s.a.w.a).
24. To tell a lie; even more grievous is a lie against or about Allah, the Holy Prophet (s.a.w.a.) or the Imams (a.s.).
25. To eat meat etc., of a dead animal, or of one not slaughtered according to the rules of sharia.
26. To drink or eat blood.
27. To eat pork, lard or any part of a pig.
29. To eat from, or use unlawful earnings; for example, piece of a dead body, or of liquor, or of gambling devices; money received in bribe or by prostitution.
30. Taking bribe for giving a judgement.
31. Not giving full weight or measure to the customer; to sell things with short weight or measure.
32. To hoard food-stuff, i.e., not selling it even when there is shortage in market - waiting for the prices to go higher.
33. To help the oppressors and unjust persons or groups; to incline towards them or to work for them.
34. Not paying wajib huquq (except when one is in straitened condition).
36. Envy, (i.e. hasad).
38. Not giving any Importance to hajj.
39. Fighting against the Friends of Allah.
40. Music - and it is the sound which is prolonged with variation of tune and crescendo, as is understood by irreligious people. (That is: Music is that sound which irreligious people accept as music).
41. Using instruments of music; listening to instrumental music.
42. Ghibat: Backbiting a mu'min by words, signs or actions. That is: To mention or disclose any actually existing physical defect, family disgrace, religious or moral shortcoming or any such defect which the mu'min concerned does not like the people to know; it makes no difference whether that mention is made by words, or actions or signs.
43. Tuhmat: To falsely accuse a mu'min of something which is not found in him or he has not done. It is a more heinous sin than Ghibat.
44. To abuse, insult or disgrace a mu'min in any other way.
45. To create enmity and mischief between the believers by conveying one's words to the other.
46. To act as pimp; to work as a go-between to gather two persons for unlawful sexual intercourse.
47. To adulterate merchandise, to make it impure or poorer in quality by adding something of less value; to pass on imitations as genuine Items.
48. To do a good deed with intention of showing to the people.
49. To repeat small sins.
50. To treat a Sin lightly because "the most grievous sin is that which the doer treats lightly':
The above list gives only the names of the major sins. If anyone wants further details, he/she should consult other books written on this subject.
Many of the above mentioned sins are also crimes punishable under Islamic penal code.