Could anyone tell me if there is a rule for the two different 'th' sounds as in think or through and this or weather?
I think this is stumping me
The "th" sound in words like "this" and "weather" is a voiced interdental fricative, represented in IPA as [ð]; the "th" sound in words like "think" and "through" is a voiceless interdental fricative, represented in IPA as [θ]. (If you're not familiar with IPA, see this: http://en.wikipedia.org/wiki/IPA .) So the difference is voicing: whether the vocal chords are vibrating or not.
The voiced sound, [ð], is generally found in grammatical words (like "the", "this", "that", "there", "though", "they", "them", "thus", "thou") and word-internally in native Germanic words (like "weather", "whether", "feather", "leather", "gather", "mother", "father", "bother", "either", "rather", "tether", "farther", "lather", "heather", "nether", "other", "brother", "brethren", "fathom").
The voiceless sound, [θ], is found word-initially in lexical words (like "thin", "think", "thistle", "thing", "thank", "thousand", "thorough", "thong", "thumb", "thud"), in all word-initial cases with "r" (like "through", "thrill", "thrall", "three", "thrice", "throng"), and in Greco-Latinate or other borrowed words (like "theory", "theist", "catholic", "catheter", "Catherine", "Luther", "Martha", "anthropic", "arthropod", "Athens", "Corinth", "lithic", "pathetic", "Seth", "Roth", "couth").
Word-finally in native Germanic words, we can generalize that [ð] is found in cases with a silent "e" (like in "breathe", "seethe", "teethe", "soothe"), and [θ] in other cases (like "breath", "north", "south", "berth", "Earth", "birth", "hearth", "fourth", "thousandth", "moth", "cloth", "tooth", "sooth", "truth"). The one exception that comes to my mind is "smooth", which uses [ð].
The voiced sound, [ð], is generally found in grammatical words (like "the", "this", "that", "there", "though", "they", "them", "thus", "thou") and word-internally in native Germanic words (like "weather", "whether", "feather", "leather", "gather", "mother", "father", "bother", "either", "rather", "tether", "farther", "lather", "heather", "nether", "other", "brother", "brethren", "fathom").
The voiceless sound, [θ], is found word-initially in lexical words (like "thin", "think", "thistle", "thing", "thank", "thousand", "thorough", "thong", "thumb", "thud"), in all word-initial cases with "r" (like "through", "thrill", "thrall", "three", "thrice", "throng"), and in Greco-Latinate or other borrowed words (like "theory", "theist", "catholic", "catheter", "Catherine", "Luther", "Martha", "anthropic", "arthropod", "Athens", "Corinth", "lithic", "pathetic", "Seth", "Roth", "couth").
Word-finally in native Germanic words, we can generalize that [ð] is found in cases with a silent "e" (like in "breathe", "seethe", "teethe", "soothe"), and [θ] in other cases (like "breath", "north", "south", "berth", "Earth", "birth", "hearth", "fourth", "thousandth", "moth", "cloth", "tooth", "sooth", "truth"). The one exception that comes to my mind is "smooth", which uses [ð].
There are no real set in stone hard and fast rules on this one - it's just one of the many inconsistencies of our glorious Language. You just learn as you go along, and the two sounds come naturally and appropriately in whichever words you utter....eg "this" as opposed to "thin" and "moth" as opposed to "mother", and so on.
Thanks for your help. I think I'll just tell my Japanese students that there are no set rules! I'll show the aadvanced students the link to the wiki site.