Learn Italian: I soldi sono molto importanti.

In English, "money" is treated as a singular noun. In Italian, however, soldi is plural (think of it literally as "coins" or "monies"), so it requires the masculine plural article i and the plural verb sono (are).

Since soldi and importanti are plural, why doesn't molto change to molti?

This is the big trick with words like molto, tanto, and troppo! Here, molto is acting as an adverb meaning "very" because it describes how important it is. When describing an adjective or a verb, it never changes its ending. It would only change to molti if it were describing a noun (for example, molti soldi meaning "a lot of money").

Could I say il soldo (singular) to talk about money?

Not usually. While un soldo originally referred to a specific old coin (similar to saying "a penny" or "a cent"), Italians almost exclusively use the plural i soldi to talk about money in general.

Why does importanti end in -i?