I'm looking to travel to Iran and I'm wondering how much Farsi I should know to be able to get by. Can I assume most people in the cities know English ? Is the signage only in Farsi or also in Latin alphabet?
I travelled there last year, firstly for a wedding and then two weeks exploring with @Stuart and another friend. We didn't use a tour, and went to Tehran, Shiraz, Yazd, Esfehan and Rasht, as well as some other smaller places near the Caspian.
The only confusing part was the dates - on our first train ticket it said the year was 1394 (I think it correlates to 2015 at the time), and someone had to explain that.
Otherwise it was fine. Generally people are super excited to meet you - the friendliest of almost any of the countries I've visited, many want to greet and meet you, practice their English. I was wary after other countries where often this means they want to sell you something, but Iranians were so genuine and eager to help and make sure you have a good time. The only time I felt ... odd was briefly while on the bus in the north of the country.
If you find someone who doesn't understand you, odds are a nearby person will speak some English and help out, especially younger people.
Of our group, I'd done a bit of a Farsi podcast training beforehand, but it was basically only greetings and the like, and while it was fun to say thank you and the like, English was very widespread in the main cities.
I totally agree with Mark, about how nice and helpful the people in Iran are and that many, but not most, speak good English and like to talk to you.
To answer the question about signage: I would say most of the signs important to tourists had an English translation. Road sign, street names, signs in Railway stations and at touristic sights. I'm just looking through my photos. Signs in smaller mosques and hand-made signs in Bazaars were in Farsi only. In smaller towns or the countryside you see less signs with English, but I never had any problem to get around. Many food products you may buy also have English on the label.
The English knowledge of Iranians is good enough to communicate with travelers, especially young ones. Iranians use many English words which enter the Farsi language, this helps you too I think:) and the signage for tourists are all written in English. Something that helps you very much is to gain information about our currency, the Toman and the Rial, cause you can decide better what to buy or about all your other expenses.