Every sentence consists of two parts.

   The Subject                    +               The Predicate.

     The rain                              Fell heavily

 
Both parts are necessary: The rain fell heavily.

If either part is missing, the part that is left is not a sentence.  

The rain!?                                      Fell heavily!?

Subject
That part of a sentence which names the person, idea or thing about which something is being said is called the subject of the sentence.

Maher is reading.   We understood the lesson.  The teacher sees them.

Predicate

That part of a sentence which says something about the person, idea or thing named by the subject is called the predicate of the sentence.

He sees the students.     The waiter knows her.    She is reading a story.

 Subject / Predicate order

1) The subject usually precedes the predicate.

The cat is eating meat.               I'm running.

2) The normal subject / Predicate order can be inverted.

Inversion is employed only for special effects in dramatic and narrative prose and in verse. It is an abnormal and artificial device.

Example :

Never to her childhood home returned The sad exile.

             Predicate                                                     Subject

Never to her childhood home returned                 The sad exile.