The “th” in “thief” is unvoiced. That means that when the “th” is pronounced, the vocal cords do not vibrate.  If anyone wants to hear how the word is pronounced, you can go here:   http://www.thefreedictionary.com/thief

The “th” in “mother” is voiced. When you pronounce the “th” in “mother,” your vocal cords vibrate. You can hear the word “mother” pronounced at: http://www.thefreedictionary.com/mother

Just click on the icon of a loudspeaker next to the words, and also click on the American and British flags.

MoTHer has just the very tip of the tongue out; try making a buzzing sound as you do it.

THief places the tongue slightly further out, and is a simple breath out; if you pay close attention, the air should rush between your top front teeth and your tongue, as if concentrated. Unlike MoTHer, where it simply feels like a vibration/buzz.

don't worry. They are identical sounds. Just stick out your tongue when you pronounce the TH sound and it does not matter if the TH is the first sound or found within the word like moTHer.

You are very welcome

Theresa

Thief is interdental s: put the tip of your tongue between the front teeth and say [s]. Mother is interdental zput the tip of your tongue between the front teeth and say [z] sometimes turns into d in colloqual speech.

They are very similar. If you do not know, the "th" sound is produced by placing the tip of the tongue between the front two teeth (I think they're called the incisors), keeping the mouth somewhat open and breathing out from the lungs. Perhaps in the word "thief" the sound is sustained for a slightly shorter duration of time.

The last sentence should be, "But I heard a difference." You forgot the indefinite article.