The end of Persian rule
The Sasanians ruled a peaceful and prosperous empire for over 400 years. Problems on the eastern and western frontiers were a worry to the kings - but ordinary people would seldom have been aware of much that was going on - their lands suffered no major invasion before the Arabs, The Mazdak movement shows that people were not happy with their rulers, but that the rulers could safely ignore their grievances. Religion was a comfort to some - even to some close to the ruling family maybe. But traditional Zoroastrianism was increasingly seen as the preserve of the ruling class, and Nestorian Christianity lacked the authority that Christians in the west could claim: they had no Pope, no access to government support.
Stress prosperity and progress - but also that it was almost entirely top down. Progressive ideas were there (Mazdakism, Church of the East) and popular with the people - far ahead of Europe and the West - but the rulers saw them as threats and suppressed them. Why did they tolerate Nestorianism after they split with Constantinople?
Though widely used the term is inaccurate, and deprecated by actual members of the church. Nestorius, archbishop of Constantinople was condemned for heresy in 431. He had no connection with the Church of the East - though they did not accept that he was a heretic. Nestorian was then used as a term to abuse Church of East by Orthodox Christians. The Church of the East was never "established" as a state religion - but was the world's largest until mass conversions by Catholics in South America after 1492.
Manichaeans followed Mani, born in Iraq in AD 216. Like some Christians they believed this world was evil, and you should give up anything that could corrupt your soul (eg drink, private property, murder, meat, vegetables, washing and sex). They were gnostics (they believed anyone could access god directly - no need for priests or a church). One of the strongest influencers of Christianity, St Augustine, started out as a Manichaean. They were systematically persecuted in the Sasanian empire - but their teachings spread along the Silk Road, and became popular in China.
Lots here: https://www.the-persians.co.uk/shapurI.3.htm
The world is often used today to refer to a stark choice between mutually-exclusive alternatives (eg Brexit!)
Sources are problematic - but it seems that Kavad ruled twice: as a young ruler he was in sympathy with Mazdak, but when he returned after a spell of imprisonment, he reverted to "normal" Sasanian practice. Mazdak's ideas - especially with regard to sharing women - caused much consternation. See here: https://www.the-persians.co.uk/kavad.htm
The Arab Conquest
Unfortunately several centuries have been compressed into a couple of paragraphs. For possible explanations for their startling success, see here: https://www.the-persians.co.uk/arab_conquest.htm
To find out details of the Arab dynasties, and the Turkish, Mongol and Turkmen regimes than followed them see the -persians website.
Khusrau and Shirin
Khusrau was believed to have had two Christians as his queens. The romantic story is a popular one in Persian literature and painting. Details here: https://www.the-persians.co.uk/ashkanians.htm
Did the Church of the east have divided loyalties? - compare attitudes to Jews and Muslims today.
The west (from the time of the Greeks) has always seen conflict with Persia as part of an epic struggle between east and west – this dates back to Homer and the Trojan War (the Trojans are obviously Greek in every possible way, but are seen by the Greeks as somehow alien). On the Parthenon, built by Pericles in Athens in the 440s BC (where did they get the money?), the sculpture decoration shows Greeks v Trojans and Greeks v Amazons (mythical female warriors – from the east), as if these were preliminary acts in the same drama. The struggles of Rome with Parthia, and the Byzantines with the Sassanids were seen in the west as a continuation of the same inevitable conflict between east and west. It's important to realise that Persia never saw it in these polarised terms. Successful invasions, apart from Alexander, were all from the east. And because of the Silk Road, Iran has looked eastwards - to Central Asia, where civilizations greatest centres were. Only after the discovery of America did the the focus of world prosperity move to Europe.
Draw a chart or map showing invasions of Iran from Europe, against invasions of Europe from Iran.
Find examples of art or architecture to show how the Turks and Mongols became "persianized".
Saudi Arabia and Iran are currently seen as deadly rivals/enemies. Can you see why?