Learn Portuguese (Portugal): Estás doido? Isso é perigosíssimo! - Are you mad? That is extremely dangerous!

We use the verb estar for temporary states. By asking Estás doido?, you are saying 'Are you acting crazy right now?' based on whatever the other person just suggested. If you used és (from the verb ser), you would be asking if they are intrinsically an insane person!

Why use isso here instead of isto or aquilo?

The pronoun isso translates to 'that' and specifically refers to something associated with the listener. In this context, it refers to the idea or action the listener just proposed. If you were pointing out a dangerous thing right next to you, you would use isto (this).

How does the ending of perigosíssimo work?

To emphasize an adjective in Portuguese, you can drop its final vowel and add the suffix -íssimo (masculine) or -íssima (feminine). Here, perigoso (dangerous) becomes perigosíssimo (extremely dangerous). Notice that the spoken stress shifts to the suffix, which is why it must have an acute accent on the first i.