Learn Italian: È una macchina grande.

You would replace the indefinite article una (a) with the feminine definite article la (the), because macchina is a feminine noun. The sentence would become È la macchina grande.

Why is it macchina grande instead of grande macchina?

In English, the adjective comes before the noun ('big car'). In Italian, descriptive adjectives like grande generally come directly after the noun they describe.

Where is the word for 'it' in this sentence?

Italian frequently drops subject words like 'I', 'you', or 'it' because the verb form already tells you the subject. Here, the verb È carries the full meaning of 'it is' all on its own.

Macchina is feminine, so why doesn't grande end in an 'a'?

While many Italian adjectives end in 'o' for masculine and 'a' for feminine, grande belongs to a different group of adjectives that end in 'e'. These adjectives use the exact same 'e' ending for both masculine and feminine singular nouns.