Grande vs. gran
The Spanish words grande and gran both signify big or large in English, but they are used in different contexts. Grande is generally used after the noun it describes, whereas gran is usually placed before the noun.
Grande
A1The word grande is used after the noun it modifies and simply means big.
La ciudad es grande.
(The city is big.)
Tengo una casa grande.
(I have a big house.)
Gran
A2The word gran, put before the noun, doesn't only mean big in size but also expresses some kind of admiration or respect.
Es una gran ciudad.
(It's a great city.)
Tengo un gran problema.
(I have a big problem.)
Summary
In summary, while both grande and gran translate into English as big, their usage depends on the context. Use grande after a noun to merely describe its size, and use gran before a noun to express admiration or significance.