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
The 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
The 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.