Learn Italian: Non ho contanti nel portafoglio.

In English, we often say we have no cash. In Italian, you simply make the verb negative by putting non (not) before it: non ho (I do not have). You do not need an extra negative word right before contanti.

Is contanti plural? In English, cash is singular.

Yes, contanti is plural in Italian. While English treats cash as a single uncountable concept, Italian treats it as a plural noun, i contanti (like saying coins or funds).

The English translation says in my wallet, but there is no word for my in the Italian sentence. Why?

In Italian, when it is obvious from the context who owns an item, you often leave out the possessive word, such as mio. Because you are saying that you do not have cash, it is naturally assumed you mean your own wallet.

Why is it nel portafoglio instead of just in portafoglio?

Nel is a combined word made from the preposition in (in) and the definite article il (the). Italian usually requires the article before a specific physical location, so nel portafoglio literally means in the wallet.