If someone did something that he knew was haram but he didn't know that it was from the kaba'ir, did he get a sin that is from the kaba'ir?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

This person is sinful since he committed what Allah, the Great and Almighty, has prohibited. But, whether such a misdeed constitutes a major sin depends on the Sharee'ah ruling: Whether he is excused for ignorance concerning such a matter or not. Ignorance does not provide one with an Islamic excuse in general.

Imam ash-Shaafi'i said: “A pubescent, sensible Muslim is not excused for his ignorance concerning the obligation of the five daily prayers, the fasting during the month of Ramadan, and performing Hajj, as well as the prohibition of fornication and adultery, killing others unjustly, theft and drinking wine and so on.”

This means that a person who commits a misdeed deserves punishment in this worldly life and in the Hereafter if ignorance does not constitute an Islamic excuse as demonstrated above.

But, if his ignorance constitutes an Islamic excuse, then he is not sinful, nor deserves punishment for the Hadeeth recorded in Saheeh Muslim that Ibn ‘Abbas said: “A man presented a container of wine to the Prophet So the Prophet told him: “Did you not know that Allah prohibited it? The man said: 'No'. Ibn ‘Abd al-Bar said: “This Hadeeth indicates that a person who is ignorant concerning the permissibility of something is not sinful.”

Allah Says (what means): {And never would We punish until We sent a messenger.} [Quran 17:15]

However, a person who can acquire knowledge but does not do his best for that is sinful.

Therefore, one has to recognize that there is a big difference between a ruling regarding a major sin and its consequences and determining whether one is sinful or not.

Allah knows best.