Learn Italian: Fermati e siediti al suolo immediatamente! - Stop and sit on the ground immediately!

In Italian, when giving an informal command (using the tu form), pronouns attach directly to the end of the verb. Since fermarsi (to stop oneself) and sedersi (to sit oneself) are reflexive verbs, the reflexive pronoun ti is added to the end of the commands ferma and siedi.

What is the base verb for "siediti" and why does the spelling change?

The base verb (infinitive) is sedersi (to sit down). It is an irregular verb. In the tu form of the present tense and the imperative, the stem changes from sed- to sied-, which is why the command is siedi (and with the pronoun attached, siediti).

Why does the sentence use "al suolo" instead of "sul suolo" for "on the ground"?

While sul suolo literally means "on the ground", al suolo (combining the preposition a + il) is the standard, natural-sounding fixed expression in Italian for actions where someone or something goes down to the ground.

How is the adverb "immediatamente" formed?

Like most regular Italian adverbs, it is formed by taking the feminine singular form of the adjective, immediata (immediate), and adding the suffix -mente (which works just like adding "-ly" in English).