Key wont crank engine but crossing the starter it will crank

I can't help much w/o year--make--model? Do you have a security system on this vehicle?
Most starters have two voltage circuits, one comes from the battery, hot all the time. The other circuit goes hot with key in the start position, may be wired through neutral switch, depending on the tranny, maybe you have a starter relay? The starter motor usually grounds through the engine block. The fact you can jump the starter motor and it cranks, sounds like the ground is ok. Use a test light, check for voltage on those two voltage terminals. Make sure tranny is out of gear and parking brake is set.
Some makes and models have shift-interlock, you can't shift it unless you step on the brake. Do the brake lites work? If not fix that first.
Some makes and models have a shift-interlock bypass button so you can press the button, then shift into neutral. Parking brake set.

The crankshaft position sensor is located on the right-side (passenger side) of the engine. At the rear of the engine, behind the starter. You need to remove the starter to remove the crank sensor. Remember to disconnect the battery first. The starter can short against the cross member.

wow no year, or even honda car model stated, amazing...but..
crossing what.?
big lug to big
or big to little terminal.
a key fact that......????????
if the starter runs then all fuses are good in the engine bay.
the main starter is not fused anyway. sorry.

if starter cranks engine, with big outside lug jumped to small terminal
that means the KEY line is failing.

the car has A/T or M/T again, i must guess.... sad that.
the M'/T the key line is interrupted by the clutch switch.
the A/T is interrupted by the PRNDL switch on the tranny
all cars can have an alarm system or other BS, that cuts
this line too. (BS like add on remote starts, etc)\

when you turn the key to start, the key line goes to 12v
and must arrive at the solenoid, if not bingo.
its obvious what is wrong, a dead key line.
find it.
a voltmeter (any made in 50 years) finds this in 30min work.

did you check the fuel pump is working first?

Do you have power on the yellow wire from the key switch to the starter relay when the switch is in the start position ?

If you check your battery cables they might be dirty with acid also the starter wire might be loose on the starter if u get a screw driver and turn your key on cross the starter out it might start that way if it does then that means u might need a starter hope it helps

There is a starter solenoid on the starter and another one on the firewall or inner fender.Follow the positive cable from the battery to the solenoid.Remove any covers that may be hiding it.Now,with the car in park or neutral and the park break set on you should be able to get it to crank by crossing the two large copper posts with a screw driver.If it cranks over now then you need to check for power to the small post with the black an red wire going to it.If you have power with the key in the start position then you need to replace that solenoid.If the is no power in that wire then you could either need a neutral safety switch or an ignition switch.If nothing happened when you crossed the posts on the solenoid then you either need a starter,the cables need to be cleaned or replaced,the nut on the starter solenoid could have loosened off,the small wire on the starter solenoid may have a bad connection or your battery does not have enough amperage in it to turn over the engine.Hope this helps.

verify that you have a fully charge battery and that you have the same amount of voltage at the battery as at the starter when the key is turned to the start position. If not repair fault at positive cable either at battery or at starter. If voltage checks good. Use a remote starter and turn key to the on position and try to crank over engine with remote starter. if engine cranks over. You have a defective starter solenoid and starter must be replaced.

Turn key to the run positio, Press on your brake and run your shifter through the gears a couple opf imes then try starting in neutral. This is for the benefit f the neutral safety switch. If still nothing , Raise the hood and and check the battery for a bad or corroded connection. If all is good, make sure car is in park. There is on the driverside fenderwell a relay that looks like a mine Roun With two small terminals and tw large terinals o opposite ides.. When you turn to crank, you put cirrent to one of the small ter minals and that causes the two large battery cables to connect and give power to the starter. If you can jump the battery side cable and the starter side post the starter will crank if nothing is wrong withe the starter.the two large posts. Use a jumper cable,NOT a coat hanger, iF THE STARTER SPINS THEN START IT WITH THE KEY. Good Luck.. Any more help let me know.

you have crossed the ignition wire some place up top before the starter! look close at the configuration up top and be sure start is grounded to block!good luck