Learn Italian: Questo coltello non è pulito.

Questo means 'this' and must match the gender and number of the noun it describes. Since coltello (knife) is a masculine singular noun, you use the masculine singular form questo.

Why does non come before the verb?

In Italian, the negative word non (not) is always placed directly before the verb it negates. So instead of saying 'is not' like in English, you literally say 'not is' (non è).

Why is there an accent mark on è?

The accent mark shows that è means 'is' (from the verb to be). Without the accent mark, the word e means 'and'. Writing the accent is very important to avoid confusing these two common words!

Would the word pulito change if we were talking about a different object?

Yes! Pulito is an adjective meaning 'clean', so its ending must match the noun it describes. It ends in -o here because coltello is masculine singular. If we were talking about a feminine object like a fork (forchetta), it would change to pulita.