Does the Samsung eternity use a 3.5 mm headphone plug?
Yes. The Eternity is a very rare breed of Samsung phones that do not require the proprietary headphone jack as with most of their other phones. You can use any standard headphone jack with the samsung eternity.
It is possible to use a 16GB Micro SDHC for your Samsung Eternity. Once you pop it in and turn on your phone, it will take a couple minutes for your phone to recognize it, simply because it has to load a massive 16GB's of information. If you intend on playing music with the card, I wouldn't recommend it. The Eternity is not very capable when it comes to handling large amounts of information, and…
If you plug in headphones with a microphone built in into a computer how can you make it work as both a microphone and a headphone?
The combined headphone/boom mic, usually have two 3.5mm plugs, color coded green and pink. Plug the green one into the headphone or speaker output and the pink one into the mic socket. If the headset has a single 3 ring,(stereo) 3.5mm plug, then it is generally not compatible with computer use. You may be able to find an adapter, that converts a stereo plug to two separate plugs.
Can headphones be used with a television that has no headphone Jack there are in and out audio jacks?
If the television has audio out jacks you can use a wireless headset, or you can connect the audio out to an amplifier that has a headphone jack. You will not be able to plug into you TV but if you have a VCR attached plug your headset into it. There is also a device that transmits the audio signal to a wireless headset.