Learn Italian: Il tetto del museo è grande. - The roof of the museum is big.

del is a combination of the preposition di (of) and the definite article il (the). Because museo is a masculine singular noun starting with a consonant, it takes the article il. When di and il are used together, they merge into the single word del.

Why is the sentence structured as "the roof of the museum" rather than "the museum's roof"?

Italian doesn't have an apostrophe-s to show possession like English does. To say that something belongs to someone or something else, you must always use the structure "the [object] of the [owner]". That is why we use the word del (of the) here.

Since tetto (roof) is a masculine word, shouldn't the adjective be grando?

Not all adjectives end in -o or -a. The word grande (big) belongs to a group of adjectives that naturally end in -e in the singular. These adjectives use the exact same form for both masculine and feminine nouns.

Why is there an accent mark on the word è?

The accent mark shows that this is the verb meaning "is" (he/she/it is). If you leave the accent mark off, the word e just means "and".