Learn Italian: Nella moda, abbinare il nero e il rosso sarebbe uno sbaglio. - In fashion, matching black and red would be a mistake.

Sarebbe means "would be." It is the present conditional form of the irregular verb essere (to be) for "he/she/it". We use the conditional here because saying it's a mistake is framed as an opinion or a hypothetical situation, rather than an absolute fact.

"Matching" ends in "-ing" in English. Why does Italian use the dictionary form abbinare?

When an action is the subject of a sentence (like "matching" is here), English uses the "-ing" form. Italian, however, uses the infinitive (the base dictionary form ending in -are, -ere, or -ire) for this. So, abbinare directly translates to the concept of "matching" or "to match".

Why is there an il before the colors (il nero e il rosso)?

In Italian, when you talk about a color as a general concept or a noun rather than an adjective describing something else, it requires the masculine singular definite article (il). You are essentially saying "the color black and the color red."

Why is it uno sbaglio and not un sbaglio?

The word sbaglio begins with an "s" followed by another consonant ("b"). For masculine words starting with this "s + consonant" sound (as well as a few other sounds like "z" and "gn"), you must use the article uno instead of un to make it easier to pronounce.