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Spinning pedals; non-functioning thumb shifter

This is a fantastic site! I'm a visual learner, and am also trying to teach myself bike repair, so I know I'll be referencing these videos as I learn. But I am at the very beginning of my learning curve, I am only just now learning the vocabulary of bikes and the basics of how they work. I'm hoping for some guidance on what I imagine are two very basic questions.

I'm not looking for detailed or exhaustive advice about making the actual repairs (I have some bike repair manuals that can help with that), I'm looking for advice along the lines of "check A first, and if that's not the problem, check B, then C."

My friend was recently given two used mountain bikes. I helped her put on the tires, clean the bike, and lube the chain -- which is pretty much the extent of my knowledge so far! Each of the bikes has a problem.
On the first one, when I get on it and start pedaling, everything is fine for the first one or two pedal rotations. But then I lose all sensation of resistance in the pedal stroke and I'm just spinning in place -- no forward motion. Give me a rundown of possible culprits (first check A, B, C, etc...)!

On the second bike, the thumb shifter on the right handlebar appears to be broken. Specifically, when I press on it, there is no clicking through of gears, rather it just depresses all the way down to the end of it's range, and when I let go, it just snaps right back up. Again, if someone could give me a list of possible reasons then I can go investigate each of the possibilities systematically.

Many thanks for any and all suggestions!
Troubleshooting really isn't that simple and easy as "Check A, B, C, etc". You have to watch and observe things to see what is going on, with some knowledge about how the bike SHOULD work. Case in point is the first bike. You need to be OFF the bike so you can watch what is going on as the pedals are moving as to have a clue of what is going on. Prop the bike up so the back wheel spins freely and then move the pedals forward and see what happens to it. No doubt from the description you wrote, something will rotate along the drive train that shouldn't rotate (my wild guess is the freehub or the freewheel), causing the wheel hub to not be moved. This should indicate the problem part.

You identified the problem on the second bike (for most part), and as you stated, you have the bike repair manuals. The shifter might be reconditionable, but just as likely it will probably need replaced.
Why is it that they make adult bikes that'll generally work for 5'9" or above, yet when you pedal these same bikes they only work for someone who is 5'4" or so?
Skyguy's right. I reckon the problem in the shifter is a broken ratchet, which means game over sadly. Perform Alex's tutorial on the shifter first (it may just be REALLY badly gummed up), but if that doesn't work bin it.
Quick tip if your shifter really is dead:

check your rear mech and DO NOT mix SRAM and Shimano shifters and rear mechs under any circumstances (SRAM and Shimano mixed on the front are fine), stick to one brand or the other,

They pull at different ratios with their rear mechs, so puts lots of stress on said rear mech and can cause the rear mech to dismantle itself very spectacularly (as my friend found out the hard way)
On bike 1, here are a couple things to check. When it starts spinning freely without resistance, stop immediately and try to look at the chain without moving the pedals. Sometimes the chain will ride up on top of the teeth in the front or back and spin without engaging. You would be able to see that the chain is sitting on top of the teeth instead of fitting in to them. However, much more likely that the freewheel/freehub has failed and will need to be replaced. Neither of these is too major to replace. But you will need some special tools and to carefully identify what is on the bike now so you can match.

On bike 2, I agree with the other posts, the shifter is probably shot. You can try putting a little chain lube into it and maybe it is just gunked up, but usually not. Also, if it is a "thumb" shifter with a single lever that sits on top of the bars. Some of these have a little switch on the side that goes between "indexed" and "friction". Make sure that it is one or the other of those positions. If it is halfway in between, it might act weird. Good luck
Thanks guys, now I know where to start!

I am not sure if you found a fix to your shifter issue.. I know all mine needed when it had the same issue was several doses of WD40.
The ratchet is on a tiny spring and often gets gummed up and stops engaging ..
several heavy shots of WD40 (spread out over 1-2 days) should fix the issue.

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