How can I differentiate between the voiced (th) and the voiceless one?
If we suppose that (th-sound) is pronounced in the voiceless form as in (think and thank) because the sound after (th) is vowel; so why we pronounce the (th-sound) in the voiced form although it followed by a vowel sound as in (the or that)?

Thanks in advance!
^_^

Question

How can I differentiate between the voiced (th) and the voiceless one?
If we suppose that (th-sound) is pronounced in the voiceless form as in (think and thank) because the sound after (th) is vowel; so why we pronounce the (th-sound) in the voiced form although it followed by a vowel sound as in (the or that)?

Thanks in advance!
^_^

Answer

hmm...I guess tis is the only way to explain
When "th" is at the beginning of a word, it is always voiceless unless it is a function word.
examples: than, that, the, thee, their, them, then, thence, there, these, they, thilk1, thine, this, thither, those, thou, though, thus, thy, thyne, thyself  - all with voiced "th" because they are all function words. 
Any other regular noun or verb which starts with "th" is voiceless
I hope that helps a bit!

Answer

Thanks a lot dear Mr. Gary 4 passing by and commenting on my post!
Yeah, i mean when do we use it in both ways (voiced/voiceless)!

BTW, thanks to Jessa's comment below; i knew how/ when to use the sound (th) correctly!
Thanks a bunch for both of you!
Cheer,
:)
Ahmed.

Answer

is your question really 'why' - as in what is the reason, or 'when' - as in what rule can I use to work out when?
'why' questions in English are rather pointless! There may have been reasons once, but it is rare that anyone knows the reason. It is just what we learn to do when we are children.