Learn Italian: Attraverso la strada e cammino sul marciapiede. - I cross the street and walk on the sidewalk.

Attraverso does double duty in Italian! It is often a preposition meaning 'through', but here it is the first-person singular ('I') form of the verb attraversare (to cross). You can tell it's a verb because it expresses the main action, matching the 'I' form of cammino (I walk) later in the sentence.

How would I apply this lesson's topic to say 'One must cross the street'?

To express a general necessity, you use the impersonal expression bisogna (it is necessary / one must) followed by an infinitive verb (the dictionary form). You would change the conjugated verb attraverso back to its infinitive and say: Bisogna attraversare la strada.

Why is it sul instead of just su (on)?

In Italian, prepositions like su almost always combine with the definite article ('the') before a noun. Because marciapiede is a masculine singular noun starting with a consonant, it takes the article il. When you combine su + il, they merge into the combined preposition sul (on the).

Is marciapiede (sidewalk) a compound word?