Won't start, new fuel pump and filter. New relay, Charged battery. Has spark new air filter turns over but will not start. Inertia switch appears to not be popped. Not sure what to do anymore. HELP please. I bought truck from side of the road not running.

 

Welcome to the forum

Should always include year of Ranger and engine size, also manual or automatic can be helpful some times, but for sure year and engine model
As 1983 thru 2011 saw many changes, lol
I will assume 1989 or later

Crank engine over a few times, pull out an easy to get at spark plug
It should be WET with "fuel"
Light the tip of the spark plug with match or lighter, should burn fast and bright
If it doesn't burn then it AIN"T FUEL in the gas tank

If its dry then watch the CEL(check engine light) on the dash
CEL should come on with key on, that means computer booted up, no computer = no start
Crank engine, CEL should go OFF as soon as engine is turning, that means crank sensor is sending a good timing pulse to computer so it will start spark and fuel injectors
If CEL stays on when cranking then no timing pulse and no start


Spray fuel into the engine, 50/50 test
See if its starts, it should if it has spark like you said
If it starts runs and dies then no fuel delivery
If it doesn't start/fire then no spark
50/50 instant results

With year and engine we can get more specific

 

My truck is 1994 with 4.0l ohv 4x4 5 speed

 

That's exactly what I have, 1994 4.0l 4x4 manual trans, mine is RED so its perfect, lol, its a joke I just like RED Rangers

You have a separate spark system, EDIS-6, they were pretty bullet proof
Computer just does the fuel system

Do the 50/50 test
It should start, run and die, if its a computer/fuel issue

I had to change the capacitors in my computer a few years back, only 3 of them, under a $5 fix, they start to leak and will cause odd issues
Getting the computer out is a pain but changing the caps isn't, if you solder or a friend does
They are the Blue ones in this picture: Ford EEC-IV
I also had odd fuelling issues

All my sensors are original except for O2 sensors which need to be changed every 12 years or so, only sensors the wear out
O2s can't cause a no start, just richer running as they get past 12 years old
Even has factory fuel pump, 400k miles, knock on wood