Can you play a super famicom on a American tv?
Yup,US and Japan both use ntsc.
But there are slight differences in the type of NTSC they say, so there could be problems just like for any other Japanese video game if you use RF. But if it's a TV from the 90s it isn't very old at all and may have AV ports. In that case there is no problem.
As someone who has one-yes they can-no modification needed. Only thing i'd advise is powering with a Genesis Model 1 power plug because the JP plugs run warm here-and i'm not sure if that's safe long term-and it seems generally advised to use the Model 1 Genesis plugs.
You can't generalise it, it depends on your TV. There are some differences in what frequencies Japanese and American NTSC use. Most Americans doesn't seem to have too much trouble with Japanese NTSC though, from what I can tell on this forum. Many people seems to use the Famicom via RF on their US TVs.
The difference between NTSC-U and NTSC-J is the RF frequency used. The composite output on both the NTSC SNES and Super Famicom are identical. Only hardware difference is the RF module. In fact, if you were to pull the RF module from a NTSC SNES and use it as a replacement for the Super Famicom module, then the Super Famicom would output RF signals on Channels 3 and 4 per NTSC-U standard.