Learn Polish: Kupiłbyś jacht, gdybyś wygrał na loterii? - Would you buy a yacht if you won the lottery?

It is the conditional mood! To build it, you take the past tense form for "he bought" (kupił) and add the conditional ending for "you" (-byś). Because it uses the masculine past tense kupił, we know the person being asked is male.

Why is there an -ś attached to the word for "if" (gdyby)?

In Polish, the word for "if" in hypothetical situations is gdyby. The personal ending for "you" (-ś) attaches directly to it, making gdybyś. The main verb that follows (wygrał) is then left in its standard past tense form. Together, gdybyś wygrał means "if you won".

How would I ask this exact question to a woman?

You need to change the masculine past tense bases into feminine ones! "She bought" is kupiła and "she won" is wygrała. By adding the same endings, the sentence becomes: Kupiłabyś jacht, gdybyś wygrała na loterii?

Why doesn't the word jacht change its ending since it is the object being bought?

Jacht is the direct object, so it is in the Accusative case. However, it is a masculine inanimate noun (a non-living thing). For masculine inanimate nouns, the Accusative case looks exactly the same as the basic dictionary form.