"Many Americans seem to entertain the illusion that Iranians are Arabs. This may be due to the fact that many people in both communities practise Islam, which I'll mention below. Another coincidence that may have contributed to this confusion is the apparent similarity of the names Iran and Iraq. It is true that the Persian language and the Arabic share the same alphabet, namely the Arabic alphabet, which was imposed upon the Iranians centuries ago. But originally Persian had its own alphabet. Anyway, in Arabic script the names of the countries are entirely different, 'Iraq' beginning
with the letter 'ain' and 'Iran' beginning with the letter 'alif'. The words 'Iranian' and 'Persian' are virtually synonymous, the former being the preferred term nowadays... The word 'Iran' is cognate with the English word 'Aryan', as the Iranians are Aryan, that is, Indo-European, while the Arabs, as is well known, are Semitic, so ethnologically there's a definite disjunction. The Indo-European languages, which probably coincide in fair measure with ethnicity, are divided into Centum and Satem groups. Centum languages further divide into Germanic, Italic, Celtic and Greek, while Satem languages divide into Balto-Slavic, Indo-Iranian, Albanian and Armenian.
Thus we find among Indo-European languages such widely divergent specimens as English, German, Spanish, French, Greek, Russian, Persian (Farsi), Hindi and many others.

"Persian has undergone many changes in the past two millennia, the most significant of which has most certainly been the influence of Arabic since the Islamic conquest of Persia in the year 650. Over the years, Persian has borrowed up to half of its vocabulary from Arabic as well as certain grammatical elements. This impact of Arabic is profound not only because of its magnitude but because the sounds and syntax of Arabic, a Semitic language, are so different than those of Persian. Since the Middle Ages, Persian has been written in a modified form of the Arabic alphabet, although in pre-Islamic times it was written in an older alphabet known as Pahlavi...

The Persian Sassanid dynasty that had been a dominant empire in the Middle East was defeated by the Muslim Arabs in the seventh century and Iranians enthusiastically embraced Islam. Persian language and culture went into a decline for several hundred years. During this time, Arabic was the language of study for both religious and secular purposes. Persian remained a spoken language only and even so was
greatly influenced by Arabic. The earlier Persian writing system was forgotten as was much of the pre-Islamic religion and folklore. Only in the tenth century did a number of Persian poets and intellectuals begin to use the Arabic writing system to write Persian." ...