Where art thou, Muse, that thou forget'st so long To speak of that which gives thee all thy might?
Spend'st thou thy fury on some worthless song,
Darkening thy power to lend base subjects light?

Return, forgetful Muse, and straight redeem
In gentle numbers time so idly spent;
Sing to the ear that doth thy lays esteem
And gives thy pen both skill and argument.

Rise, restive Muse, my love's sweet face survey,
If Time have any wrinkle graven there;
If any, be a satire to decay,
And make Time's spoils despised every where.

Give my love fame faster than Time wastes life;
So thou prevent'st his scythe and crooked knife.

—“Sonnet 100,”
William Shakespeare

What is the primary purpose of the couplet in this sonnet?

To express that the author's 'Muse' (or inspiration) is gone

Explanation:

"Where art thou, Muse, that thou forget'st so long

To speak of that which gives thee all thy might?

Spend'st thou thy fury on some worthless song,

Darkening thy power to lend base subjects light?"

I can sorta guess that this is asking something like "Where are you Muse, did you forget about me for so long?"

the tone of the passage is sympathetic to the forbidden love of romeo and juliet. the nurse plays a significant role in the play. she is responsible for making their meeting again possible. additionally, there is a sense of warning in the choice of words of the nurse. she seems to be authoritative when she calls romeo "scurvy knave", and at the same time the fact that she addresses to him as "sir" reflects respect. the nurse is "vexed", this word describes her confused state of mind. on the one hand, she finds romeo a fine and eloquent man, but on the other, it is thought that man's nature forces men to try to trick women.  

coin toss of the tosses to the toss