And tell me if they are correct and with 15-20

The complete subject is everything before the verb, and the simple subject is specifically the noun the verb refers to.

The complete predicate is everything after and including the verb, and the simple predicate is the verb.

(For #1-14, I only mentioned anything incorrect.)

4. The simple subject is actually dozens not paintings.  The subject cannot be within a prepositional phrase, which is a phrase that begins with a preposition.  The word "of" is a preposition and starts the prepositional phrase "of Chang's paintings", so paintings cannot be the subject, it is dozens.

5. The simple subject is guide not floor.  You correctly identified the simple predicate as walked, and the simple predicate refers to the simple subject; the guide walked, not the floor.  Since guide is the simple subject, then the complete subject should be our guide, and the complete predicate should be across the marble floor walked.

6. The simple subject is just guide, not "the guide".  "The" is an article, a type of adjective, and adjectives are never included in the simple subject.  

10. The simple subject is fan.  See explanation for #5.

12. The simple subject is picture.  See explanation for #4.

13. The simple subject is artwork.  See explanation for #4.

15. To make this sentence easier to work with, rearrange it in the form of a statement: "The group did see all of his paintings with flowers and still lifes".

Simple predicate: did see (these are the only verbs in the sentence)

Simple subject: group (group did see)

Complete predicate: did see all of his paintings with flowers and still lifes.

Complete subject: The group

16. Rearrange this sentence: "A picture of grapes is included among the images".

Simple predicate: is included (verbs)

Simple subject: picture (picture is included)

Complete predicate: is included among the images.

Complete subject: A picture of grapes

17. Simple predicate: hangs (verb)

Simple subject: painting (painting hangs)

Complete predicate: hangs nearby

Complete subject: A colorful landscape painting

18. Simple predicate: are renowned (verb)

Simple subject: landscapes (landscapes are renowned)

Complete predicate: are renowned for their technical achievement.

Complete subject: His landscapes

19. Simple predicate: are (verb)

Simple subject: mountains (mountains are)

Complete predicate: are beautiful.

Complete subject: The mountains in this painting

20. Simple predicate: used (verb)

Simple subject: Zhu Qizhan (Zhu Qizhan used)

Complete predicate: used impressionist techniques in his work.

Complete subject: Zhu Qizhan

Hope this helps!!

brutus overlooks that the majority of the crowd served julius caesar the military general. he appeals to their citizenship as romans by calling them first by that name in his salutation: "romans, countrymen, lovers." mark antony, however, speaks in terms understood by the military, by calling them first by, "friends, romans, countrymen." he recalls not only caesar the general but caesar the friend, whose body they see covered with stab wounds.