How can the Nurse be viewed as a foil to Juliet?

In Shakespearean drama a foil is a character who is used as a contrast to another character. This contrast makes the particular qualities of each character stand out. In Romeo and Juliet Shakespeare uses the Nurse as a foil to Juliet and Mercutio as a foil to Romeo. 

In Act I, Scene 3, we first meet Juliet and the Nurse. Juliet is a thirteen-year old girl who knows little of the world. In the preceding scene, her father, Lord Capulet says,

My child is yet a stranger in the world.
She hath not seen the change of fourteen years.

She's very innocent as opposed to the Nurse, who is much older and experienced. The Nurse is lower class as opposed to Juliet, whose family is very wealthy. One technical difference in the play is that, since she is a commoner, the Nurse rarely speaks in iambic pentameter (ten stressed and unstressed syllables per line) and she never uses couplets (two consecutive rhyming lines). Here she talks about Juliet's age:

I’ll lay fourteen of my teeth (and yet, to my teen
be it spoken, I have but four) she’s not fourteen.
How long is it now to Lammastide?

This contrasts with Juliet, who always speaks in iambic pentameter and often uses couplets. For example, Juliet explains how she will look at Count Paris:

I’ll look to like, if looking liking move.
But no more deep will I endart mine eye
Than your consent gives strength to make it fly.

The biggest contrast between the two is how they think of love. The Nurse defines love with its physical and sexual connotation. She uses sexual innuendo and the physical aspects of marriage when she's talking to Juliet about Count Paris. When Lady Capulet and the Nurse try to convince Juliet she should be interested in Paris, the Nurse describes Paris physically:

A man, young lady—lady, such a man
As all the world—why, he’s a man of wax.

And a little later she says,

No less? Nay, bigger. Women grow by men.

referring to pregnancy. Later, in Act II, Scene 5, she speaks of Romeo, not as the ideal love, but as a physical specimen. The Nurse says,

Though his face be better than any man’s, yet his leg
excels all men’s, and for a hand and a foot and a
body, though they be not to be talked on, yet they
are past compare.

Rather than discussing Romeo as a spiritual match for Juliet she dwells on the physical and sexual. She tells Juliet she must find a rope ladder so that after the couple is married Romeo can crawl into Juliet's room for the honeymoon. In a reference to the actual sexual act, she says,

I must another way,
To fetch a ladder by the which your love
Must climb a bird’s nest soon when it is dark.
I am the drudge and toil in your delight,
But you shall bear the burden soon at night.

Juliet, on the other hand, is more interested in ideal love and the beauty of mutual devotion between a man and a woman. Romeo is perfect for her. In his first words to her he compares himself to a pilgrim worshipping at the altar of her beauty. These words hook Juliet into love at first sight, or first sonnet, as the opening fourteen lines between the two youngsters in Act I, Scene 5, is a Shakespearean sonnet. Juliet is immediately attracted as Romeo holds her hand, and calls it divine:

Romeo:

If I profane with my unworthiest hand
This holy shrine, the gentle sin is this:
My lips, two blushing pilgrims, ready stand
To smooth that rough touch with a tender kiss.

Juliet:

Good pilgrim, you do wrong your hand too much,
Which mannerly devotion shows in this;
For saints have hands that pilgrims’ hands do touch,
And palm to palm is holy palmers’ kiss.

This devotional language continues in Act II, Scene 2, as Romeo and Juliet declare their love for each other and decide, despite the bitterness of the feud between their families, that they will get married.

The final contrast between the Nurse and Juliet comes in Act III, Scene 5, when the Nurse advises Juliet to forget Romeo and heed the wishes of her father by marrying Count Paris. The Nurse cannot fathom the depth of feeling Juliet has for Romeo and sees him as just another man. She makes the point that Paris is a better match physically:

O, he’s a lovely gentleman!
Romeo’s a dishclout to him. An eagle, madam,
Hath not so green, so quick, so fair an eye
As Paris hath.

This alienates Juliet from the Nurse as she is shocked by he confidante's words. When the Nurse leaves, she symbolically breaks her bond with the woman. She says,

Ancient damnation, O most wicked fiend!
Is it more sin to wish me thus forsworn
Or to dispraise my lord with that same tongue
Which she hath praised him with above compare
So many thousand times? Go, counselor.
Thou and my bosom henceforth shall be twain.