Learn Russian: Здесь самая широкая улица в городе.

In Russian, you can form the superlative ("the most..." or "-est") by using the word самый followed by a regular adjective. Because улица (street) is a feminine noun, both words take their feminine forms: самая широкая (literally: the most wide).

Why isn't there a word for "is" in this sentence?

Just like in beginner sentences, Russian usually drops the verb "to be" (is, am, are) in the present tense. Literally, this sentence translates as "Here the most wide street in the city."

Why is the word for city spelled в городе instead of в город?

Городе is in the prepositional case because it describes a location (answering the question "where?"). The basic dictionary form is город, and we add an -е at the end because it follows the preposition в (in) to indicate location.

I noticed широкая starts with ши. Doesn't the letter ш usually sound hard?

Yes, ш is always pronounced as a hard sound. However, because of a strict Russian spelling rule, you can never write the letter ы after ш. You must write и instead, even though you still pronounce the syllable like a hard "shy".