Theory:

Metaphor:
A metaphor is a figure of speech that makes an implicit, implied, or hidden comparison between two things that are unrelated, but which share some common characteristicsMetaphors are used to dramatize thoughts to emphasize and elevate the expression to a higher intensity.
Example:
1. Jack was fishing for compliments - In the real sense, Jack cannot be casting a lure to catch compliments from a river! It just means he was looking for compliments.
2. John was so hungry that he could eat a horse - In the real sense, it means he was extremely hungry, and he could eat a lot of food, but not a whole horse!
3. The sun is a golden ball - In the real sense, one cannot throw and play with the golden ball that the sun is! 
Metaphors used in the poem "A thing of beauty":
  • bower - a calm, pleasant shade
  • sweet dreams - happy dreams
  • flowery band - to bind us to earth
  • pall - a cloth covering/a cloud of dust, smoke
  • endless fountain of immortal drink - a shower of blessings of good health
Alliteration:
Alliteration is the occurrence of the same letter at the starting of two or more words in a single line, in a poem.
Example:
1. The slender smiling girl...
2. The song of sweet birds...
3. Black bug bit a bear...
4. Practise the piano...
What are the uses of alliteration?

1. It creates a rhythm, similar to rhyming words.
2. It emphasizes the importance of phrases.
3. Mostly used in tongue-twisters.
Alliteration used in the poem "A thing of beauty":
  • A flowery band to bind us to the earth,
  • Spite of despondence, of the inhuman dearth
  • Some shape of beauty moves away the pall.
  • From our dark spirits. Such the sun, the moon,
  • Trees old and young, sprouting a shady boon
  • For simple sheep; and such are daffodils
  • That for themselves a cooling covert make
  • 'Gainst the hot season; the mid forest brake,
  • Rich with a sprinkling of fair musk-rose blooms:
Imagery:
Imagery means using imaginative language to represent objects, actions, and ideas in such a way that it appeals to our physical senses. A writer helps the reader to visualize or to see in their minds what is being described. The images should appeal to one or more of the five senses: sight, hearing, touch, taste or smell.
Example:
1. It was dark and dim that night.
    The words “dark” and “dim” are visual images; (that appeal to our eyes).
 
2. The kids were screaming and laughing in the classroom.
     “Screaming” and “laughing” appeal to our sense of hearing; (that appeal to our ears).
  
3. Rani whiffed the fragrance of the fresh rose blossoms.
    “Whiff” and “fragrance” evoke our sense of smell; (that appeal to our nose).   
What are the uses of imagery?
 
1. The concept is made easier to understand, as the reader visualizes it.
2. It makes the concept easier to remember.
Imagery used in the poem "A thing of beauty":
  • A flowery band to bind us to the earth
  • Some shape of beauty moves away the pall
  • From our dark spirits. Such the sun, the moon,
  • Trees old and young, sprouting a shady boon
  • With the green world they live in; and clear rills
  • That for themselves a cooling covert make
  • Rich with a sprinkling of fair musk-rose blooms
  • An endless fountain of immortal drink
  • Pouring unto us from the heaven's brink