Farsi, Dari, and Tajik
Persian is spoken across three countries under three names, and they are closer than many people expect.
Persian has three main standard varieties: Farsi in Iran, Dari دری in Afghanistan, and Tajik in Tajikistan. Linguists treat them as one language with regional forms.
How alike are they?
Farsi and Dari are highly mutually intelligible, much like British and American English. A speaker of one can read and understand the other with ease, allowing for accent and some vocabulary differences.
What sets them apart
Dari keeps some older pronunciations and vocabulary that Iranian Farsi has changed. Each variety has borrowed differently: Iranian Farsi from French and Arabic, Dari with its own regional terms, and Tajik heavily from Russian.
The big difference: script
Farsi and Dari both use the Perso-Arabic script. Tajik, however, is written in the Cyrillic alphabet, a legacy of the Soviet era, which makes it look very different on the page despite sounding familiar.
If you learn Farsi, you have a strong head start on Dari and a real foothold in Tajik once you adjust to its script.