Learn Farsi: قوها تند نمی‌دوند، در عوض خیلی زیبا هستند. - Swans do not run fast; on the other hand, they are very beautiful.

You are exactly right that avaz means 'exchange' or 'substitute'. When we use the compound phrase dar avaz, it literally means 'in exchange for that'. In this sentence, it acts as a coordinator of contrast meaning 'on the other hand' or 'to make up for it'—the swans offer beauty in exchange for their lack of speed!

Doesn't the word tond usually mean 'spicy'?

It can! tond is a very versatile word. When talking about food, it means 'spicy' or 'hot'. But when describing motion or action, like with the verb davidan (to run) in this sentence, it means 'fast' or 'quick'.

Why do the verbs nemīdavand and hastand have the plural ending -and? Don't animals sometimes take singular verbs?

Good memory! While plural inanimate objects must take a singular verb in Farsi, plural animals give you a choice. You can use a singular verb to talk about them as a general group, but it is also very common and perfectly natural to use a plural verb like hastand, especially when treating them as active, living subjects.

I know the infinitive is davidan (to run), but how exactly is nemīdavand formed here?

The present stem of the verb davidan is dav. To make a present tense verb, we use the prefix mi-, and to make it negative, we add ne- right before that. Put it together with the 'they' ending -and, and you get (they do not run).