After attending many NSAI meetings I will share with you the basics I have learned
for commercial songwriting....

Be aware of where your verses, chorus or bridge are and very importantly keeping 
every verse melodically identical, keeping the same number of lines in each verse, 
and trying to have close to the same amount of syllables in a line in one verse matching 
the line in verse two....example (from "Carrying Your Love With Me- George Strait)

Verse 1, line 1&2:
Baby all I got is this beat up leather bag
And everything I own don't fill up half

Verse 2, line 1&2:
On a lonely highway stuck out in the rain
Darlin all I have to do is speak your name

Note that the lines are really similar in syllables and you sing them the same 
melody AND the rhymes are also matching each other in both verses. You will find 
this in every single song ever written. Some lines will have more syllables than 
others but some words you can stretch out singing to fit the melody.

A chorus is the foundation of a song idea and what the song is about, also the "hook", 
the catchy part that makes you wanna hear it again and again. The verses must build up 
to the chorus...the chorus is supposed to be the climax of the song. Some songs such as
"I'm Movin On" by Rascal Flatts really don't have an actual chorus, a chorus is not 
imperative it's just most common. "I'm Movin On" does however have a "Bridge" instead 
of chorus, and is able to take it's place because the verses build up to it and it is 
a powerful statement. The purpose of the bridge is to add something different to a song 
that would otherwise be 3 verses, they needed something different in the middle to shake 
up the melody and make the song really interesting.

What exactly is a "bridge"? It is a part of the song completely different from all the 
rest (not a verse, not a chorus) Most times it is an overall reflective thought summing 
up the whole song. Here is an example of one of my favorit