My sister needs help with this, but I don’t understand Spanish very well. So, please help me!

sentences vary by structure as well as by function. there are compound, complex, and compound-complex sentences.

simple sentence

i like to read.

a simple sentence has one independent clause.

compound sentence

i like to read, and i also like to write.

a compound sentence includes more than one independent clause, connected by a coordinating conjunction (and, but, for, nor, or, so, or yet).

complex sentence

i prefer to read books that are bestsellers.

a complex sentence includes one independent clause and one or more dependent clauses. (a dependent clause is one that cannot stand alone.) types of dependent clauses include relative clauses (i kicked the boy who pinched me), adverb clauses (i am going home now because i have a curfew), and noun clauses (i don't know what to do next).

compound-complex sentence

although i prefer to read current bestsellers, i do like to read old agatha christie mysteries, and i also like some 20th-century science fiction.

compound-complex sentences take a bunch of clauses (multiple independent clauses, and one or more dependent clauses) and toss them together like a salad. although these sentences take practice to write in a grammatically correct fashion, they can hold a tremendous amount of meaning, and are common in academic writing.

figurative language is a resource that fiction writers use to make better descriptions of what they want to write about or express. they can improve their stories, as they can elaborate more vivid and colorful sentences. this way, the reader will be able to have more detailed images in his/her mind.

places, objects, people and even feelings or emotions can be described with a lot of detail to metaphors, similes and personifications. making different kinds of comparisons to make a description more specific, as well as assigning human qualities to non-human objects (and vice-versa) can be very useful for science fiction writers.  

on the other hand, alliterations allow writers to elaborate more entertaining and creative sentences. it consists in using a sequence of words that start with the same consonant letter. they can be used to emphasize something important the writer wants to express, or just as a fun literary device.