Please explain why the sentence, The flat creek bed was made of warm soft mud is neither a compound subject nor a compound predicate?

Answer 1

Please note the correct spelling of "grammar."

That sentence has only one subject (creek bed) and one verb (was made). It's a simple sentence.

Here's a sentence with a compound subject:
Jack and Jill went up the hill. (2 subjects with 1 verb)

Here's a sentence with a compound verb:
They ran and jumped all the way home. (1 subject with 2 verbs)