Why did Bassanio borrow money from Antonio, and how does this create a conflict between Antonio and Shylock?
As was mentioned in the previous post, Bassanio has squandered his money and asks Antonio for a loan in order to travel to Belmont to win Portia's hand in marriage. Portia is a wealthy heiress who is bound to her father's will. Many suitors from around the world travel to Belmont in hopes of marrying Portia. Bassanio understands that he will need money to outfit ships, buy various gifts that will impress Portia, and compete with the other suitors. Unfortunately, he does not have the money to do so which is why he asks his close friend, Antonio, for a loan. Antonio is a wealthy merchant who agrees to finance Bassanio's journey to Belmont. Antonio is a gracious, loving friend who selflessly loans Bassanio money, even though his investments are tied up overseas. Antonio then borrows money from Shylock to give to Bassanio. The conflict is created when Antonio's investments do not return on time and he is forced to forfeit on Shylock's loan. Unfortunately, Antonio agreed to give a pound of flesh to Shylock if he was not able to pay back the loan. Shylock and Antonio end up going to court over the forfeiture and Antonio fortunately wins the case.
Bassanio needed to borrow money in order to woo the wealthy and beautiful Portia, a noblewoman with an estate. To woo such an important woman, Bassanio needed to dress well, prepare gifts for her, travel to her estate, and generally spend money to present himself in the best possible way.
Unfortunately, Bassanio was careless with money, and did not have enough to properly woo Portia. Therefore he asked Antonio, his close friend and a generally successful merchant, to loan him some money.
Antonio didn't have any actual money, because he invested it all in some ships (he was a merchant, after all--investing is how he made money). Antonio expected his ships to return soon, so he decided to help Bassanio by going to Shylock, a professional moneylender, to borrow money until his ships returned.
In other words, Bassanio didn't borrow directly from Shylock -- he borrowed from Antonio, who borrowed from Shylock -- and he did it in order to try to win Portia.