Questions & Answers about Это самый большой город в мире.
Why do we use самый большой instead of a single word for "biggest"?
Why are самый and большой both in this specific form?
Because самый acts exactly like an adjective. It has to match the noun it describes in gender, case, and number. Since город (city) is a masculine noun in the nominative (dictionary) case, both самый and большой take their standard masculine nominative endings.
Why is the word for "world" spelled мире here instead of мир?
Because it is indicating a location. When talking about location after the preposition в (in), Russian uses the prepositional case. For most masculine nouns like мир, that means adding an -е to the end, making it в мире.
Where is the verb in this sentence? How do we say "This is"?
Russian usually drops the verb "to be" in the present tense. The word Это works as the subject "this," and the "is" is simply understood from the context.