Learn Italian: I dettagli che tu vedi andrebbero bene sul vestito. - The details that you see would go well on the dress.

Andrebbero is the present conditional of andare (to go). Andare has an irregular stem in the conditional tense: it drops a vowel to become andr-. Since i dettagli (the details) is a plural 'they' subject, we add the ending -ebbero, making it andrebbero (they would go).

What does the phrase andrebbero bene mean in this context?

The expression andare bene is very common and means to go well, to be fine, or to fit. In the conditional tense here, andrebbero bene translates to would go well or would look good on the dress.

Is the word tu necessary in che tu vedi?

Strictly speaking, no. The verb ending in vedi already tells us it means you see. Including tu here just adds a little emphasis, highlighting that these are the details that you specifically are looking at.

Why is it sul vestito instead of su il vestito?

In Italian, the preposition su (on) always merges with the definite article that follows it. Because vestito is a masculine singular noun starting with a regular consonant, it takes the article il. When su and il combine, they form the single word sul (on the).