Learn Polish: Gdyby nagle został milionerem, założyłby szalony biznes. - If he suddenly became a millionaire, he would start a crazy business.

In Polish, the conditional particle -by loves to attach to the first available conjunction or particle in a clause. In the 'if' clause, it attaches directly to gdy to form gdyby. In the main clause, there is no conjunction, so it attaches to the verb, making założyłby.

Why is the word for millionaire written as milionerem instead of the basic form milioner?

The verb zostać, when it means 'to become', requires the noun that follows it to be in the Instrumental case. Since milioner is a masculine noun, it takes the -em ending.

I learned that zostać means 'to stay'. How do I know it means 'to become' in this sentence?

You can tell by what follows it! Zostać means 'to stay' when followed by a location, but it means 'to become' when followed by a noun or adjective describing a new state or profession. This new state is almost always in the Instrumental case, like milionerem here.

I know założyć means 'to put on' clothes. Does it have another meaning here?

Yes! While założyć is commonly used for putting on clothes, it also means 'to start', 'to found', or 'to establish' when talking about a business, an organization, or a bank account.