What are the subplots of The Merchant of Venice?
There are certainly a few important subplots in the play.
The most important plot involves Bassanio's loan dilemma with Shylock. When Bassanio is in need of money to court the beauteous Portia, he borrows 3,000 ducats from Shylock. Bassanio's friend, Antonio, acts as his surety. However, the wily moneylender is Antonio's enemy; he stipulates that Antonio must forfeit a pound of his flesh should he fail to repay the loan in three months. The main action in the play involves Bassanio and Antonio's attempts to save Antonio after the latter's ships are lost at sea (and with it much of his tremendous fortune).
Subplots of the play include the following:
1) The love stories of Portia/Bassanio, Jessica/Lorenzo, and Nerissa/Gratiano
In the play, Bassanio is enamored with the beauteous Portia. However, Portia's late father's will states that the successful suitor must prevail over the riddle of the three caskets (one gold, one silver, and one lead) or forever remain a bachelor. Portia has many suitors, but none of them succeed or desire to abide by the terms of the will. The Prince of Morocco chooses the gold casket, while the Prince of Arragon chooses the silver. Bassanio is the only one who chooses the correct casket (lead).
After Bassanio chooses the right casket, Portia gives him a ring to symbolize their commitment to each other. She makes him promise that he will never part with it. Meanwhile, Portia's lady-in-waiting, Nerissa, professes her love for Gratiano and also bequeaths him a ring.
The third romance in the play concerns Jessica and Lorenzo. When Jessica elopes with the latter, Shylock is furious. He thinks that Jessica has betrayed him.
2) The "ring" test used by Portia and Nerissa to gauge Bassanio and Gratiano's loyalties.
Later in the play, Portia (disguised as a doctor of law) presides over Antonio and Shylock's court case. Assisted by Nerissa (disguised as a clerk), she successfully maneuvers the proceedings to expose Shylock's vehement hatred for Antonio. The case is decided in Antonio's favor after Portia finds Shylock guilty for conspiring to take the life of a Venetian citizen. To pay Portia for her services, both Bassanio and Gratiano give up the rings given to them.
Since Portia and Nerissa are disguised, both men do not know that they have given their rings back to their wives. Later, Portia and Nerissa have a little fun at Bassanio and Gratiano's expense when they demand to see the rings.
One subplot is the casket plot for Portia's suitors. Portia's father will not allow any man to marry her unless they can correctly choose the casket that contains Portia's portrait. Until Bassanio shows up, all potential suitors have failed at this test.
A second subplot could be the contract that exists between Antonio and Shylock. Antonio needs to borrow some money and approaches Shylock, the moneylender. Shylock loans the money out interest-free, but demands a pound of flesh if Antonio cannot repay the loan.
A part of that plot would be the entire court case. Portia dresses up like a man and is Antonio's defense attorney.
A third subplot would be the multiple relationships and marriages going on: Bassanio and Portia, Nerissa and Gratiano, and Lorenzo and Jessica.
The last subplot is the ring subplot between Portia, Nerissa, Bassanio, and Gratiano.