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2000 alero

my 2000 alero was WAY low on oil so I added a quart then drove for about 45 mins at 40 mph the car seemed fine. the following morning when I went to start it the car turned over but would not start when it finally started the engine knocks and it is putting out a lot of white exhaust. obviously I need to see a mechanic but what might be wrong. my neighbor said one of the cylinders isn’t doing what ever it is supposed to do he said it passed a compression test but I don’t know what that means

I have no idea what you’re neighbor means when he said it passed a compression test. A compression test has to be done with a compression gauge, and its not a “pass/fail” kind of thing. You get a set of numbers in pounds per square inch of pressure. Those should be at a minimum level that varies a bit by engine but think of being reasonably happy at about 150psi (for an old car) and really happy at 180psi. But one of the most important things is not a lot of variation between cylinders.

Anyway - a lot of white smoke out of the exhaust usually means a blown head gasket or some similar really bad problem. That would just mean a new head gasket - expensive but not fatal. But with the knocking I’d say this engine is probably cooked.

You need an evaluation of the head gasket for coolant issues. This seems like the oil issue may be a distraction. White smoke for more than 30 seconds is a big problem. If the knock has white smoke at the same time one of two things are happening. Smell the exhaust for a second. Is it more gas smell or kinda sweet and moist.

A lot of white exhaust means that either water or oil has been introduced into combustion. Your neighbor I guess did a compression test on each cylinder and found one that failed the test. This generally means that that a piston has blown rings: a piston’s rings are just a pair of tentioned metal bands set into each piston. In order to run for more than five seconds, each piston must be well lubricated. But if the rings fail, oil will get into the combustion chamber.
You say that the car was way low on oil but you added 1 quart of oil. Way low means you needed to add more than one quart of oil. I would guess that you damaged the rings of one cylinder, and that now oil is getting into the gas-oxygen mixture of at least one combustion chamber. That will produce a lot of white smoke out of the exhaust.

At the same time, this could just be a cracked head or a blown head-gasket, which might let radiator fluid into the combustion chamber, with the same result plus overheating.
If the car is overheating (which is consistent with the engine knocking) then this is your problem.
You do need a mechanic’s help, but either way the repair may not be worth the cost for this car.
In the future, however this ends up: in any car, check the oil and all other fluids at least once each month, and check the tire pressures as well.