Learn Farsi: رفتن به تهران عالی است.

رفتن is an infinitive, which you can easily spot by its -tan ending. In Farsi, infinitives act like nouns, so they can be used as the subject of a sentence. Because of this, رفتن can translate to "to go" or "going" depending on what sounds more natural in English.

How do I find the present stem for رفتن based on the rules we just learned?

While finding the past stem is easy (just drop the final -an), extracting the present stem is often irregular and requires memorization. If you take off the -tan from رفتن, you might expect the present stem to be "raf", but the actual present stem you will memorize for this verb is rav (or ro in everyday spoken Farsi).

Do I always need to use به after رفتن?

Yes, just like in English, the action of going usually needs a preposition to show the destination. به means "to", so رفتن به تهران matches the English structure "going to Tehran" perfectly.

Why does است (is) come at the very end of this sentence?

Farsi has a very consistent rule where the main verb of the sentence almost always comes at the end. Here, the subject is the entire phrase رفتن به تهران (going to Tehran), followed by the description عالی (excellent), and finally wrapped up with the verb است (is).