In “the monkey’s paw,” a stranger comes to the door. how does this scene contribute tension to the rising action of the story?

Because you might assume that the person at the door is dangerous and there to do something bad to them. Or it's a phantom.

In der Kurzgeschichte "The Monkey's Paw" von WW Jacobs kommt ein Fremder zur Tür. Die Art und Weise, wie diese Szene zur steigenden Handlung der Geschichte beiträgt, ist "Der Fremde bringt die traurige Nachricht, dass der Sohn der Weißen bei einem Arbeitsunfall ums Leben gekommen ist."

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.