Learn Farsi: به آن سگ خطرناک دست نزنید.

The verb for "to touch" is the compound verb dast zadan. To make it a negative command, we keep the word dast, then add the negative prefix na- to the present stem of zadan (which is zan), and finish with the formal or plural "you" ending -id. This gives us dast nazanid.

Why does the sentence start with به (be)?

In English, you just "touch something," but in Farsi, the compound verb dast zadan requires the preposition به (be, meaning "to"). You are literally saying "do not hit a hand to that dangerous dog."

Is there a hidden ezafe (linking sound) in آن سگ خطرناک?

Yes! There is an invisible ezafe (an "e" sound) linking the noun to its adjective: sag-e khatarnāk (dangerous dog). However, the word آن (ān, meaning "that") comes before the noun and never takes an ezafe. The whole phrase is pronounced ān sag-e khatarnāk.