My bike
 

Currently i have a free spirit at1800 bike. I wanted to know that recently my left pedal has been lose and ends up coming off. When i tighten it after a little while of riding the whole pedal as usaul comes off. It comes off because the nut falls out which makes the pedal fall off. I was wondering how i could fix this problem. Sorry for the bad grammar.

Go to a bike shop; they should be able to replace either the pedal or the crank (or both). One or both of them is stripped out. This is a simple and quick repair.

A) The tread on the spindle that the nut attaches to is stripped, which means you need to replace the pedal. They probably get sold in pairs. Walmart might even have them. Make sure you get the right size, or it will not fit into the crank arm.

B) The tread on the crank arm is possibly stripped. The Pedal spindle is steel, and most likely the crank arm is alloy. If both are steel...somebody probably "cross-treaded" when they put the pedal in. If you need to replace the crank arm...does the tread in the hole where the pedal goes look "stripped" of tread? ...that could be expensive, unless you can do it yourself with a salvaged part. You usually need to use a crank puller to separate the crank arm from it's spindle.

On a Free Spirit, it may well be they have Ashtabula cranks (one-piece crank). If the threads on the pedal are stipped, no problem replacing. Good set of pedals to fit these are $20. If the crank's threads stripped, you'll need a new crank. Also cheap but a pain the get in and out. This is IF you have a one-piece crankset.