- What is Shirk - Polytheism

1)  For a person to become a Muslim it is necessary for them to believe Islam as the true religion and does not reject any of it's necessary aspects, and also does not have beliefs contradictory or against the religion's necessary aspects (Zaruriyat-e-Deen). 

Whether the person does not have the knowledge about all the religion's necessary aspect, i.e. even if he is a complete and utter illiterate he must believe in Islam and in the messenger of Islam and not have any beliefs against the necessary aspects of Islam (Zaruriyat-e-Deen). 

Whether he cannot pray the Kalima (submission into Islam) properly he is still a Muslim and not a Kafir. Therefore, if he misses Salaah, Siyam, Hajj etc. he will be a grave sinner but will remain a Muslim. This is because deeds are not a part of faith.

2) Whatever is without doubt Haram (forbidden) and to believe it as Halal (allowed), and to believe something Haram which is no doubt Halal is Kufr, when there is no suspicion of a particular thing being Halal or Haram or the person knows this.