I don't understand this question :

what were the circumstances of hamlet sr.'s death?

are they asking what triggered his death?

or what was the situation that triggered hamlet's death?

The question is asking how the elder Hamlet died. You'll find this question answered when the ghost of the elder Hamlet speaks with his son (Hamlet) in the first act.

thank you so much! you're helping us a lot :)

You're very welcome.

In addition...

Here is a wonderful site:
http://nfs.sparknotes.com/

Find the title and choose the part you want to read. You'll find it in two columns -- Shakespearean English in one column, modern day English in the other.

The question is asking for the circumstances or situation surrounding the death of Hamlet Sr., who is a character in Shakespeare's play "Hamlet." To answer the question, you need to review the play and analyze the events leading up to the death of Hamlet Sr. Some key facts to consider are that Hamlet Sr. was the former King of Denmark and the father of the play's protagonist, Prince Hamlet. Hamlet Sr. died before the play's events, and his death sets off a chain of events, including the revenge plot that defines the narrative. To understand the circumstances of his death, pay attention to the dialogue, references, and reactions of the characters throughout the play.