There is no rule, you just have to memorize the spellings, just like with English words that have the same sound but different spellings: write/right, red/read, no/know; ceiling, cite, cemetery, with a "c" rather than an "s" though both letters make the "s" sound, knee not nee, phone not fone, etc. At one time, the two letters were pronounced differently. This distinction is not observed by most people anymore. ט was/is pronounced in the throat. ת even had/has more than one pronunciation depending on its position in a word and the historical time and location of the Hebrew speakers. It is interesting linguistic history but we don't have to know it, or learn it, to study standard Modern Hebrew.
@benton.1 Thanks a lot for your reply. I am actually tracing the original Hebrew pronunciation not the modern Hebrew. For example I found out that the "V" sound never existed in Ancient/ Biblical Hebrew. It has always been "B" sound.
So for example it is not Avraham but Abraham and not Tel Aviv but Tel Abib.
I am also passionate about the original sound of Hebrew. Knowing the history of a language helps a lot with spelling. I am no longer surprised by the various sounds the letter vav "makes" after I learned that originally it was a sound close to English "w" like in "wine". I also discovered that most of the "hidden" Hebrew wovels appear by themselves when you try hard to pronounce all the consonants in a word one after another without wovels at all. I also practiced very proper way of pronouncing the consonants. Now I am able to write a word I hear pretty much correctly even without understanding what it means! There are great resources in the Internet as to ancient Hebrew.
- Ancient Hebrew Alphabet Pronunciation https://www.youtube.com/watch?v=O7RIAPosxEI
- The Real Story of Hebrew Pronunciation https://www.ou.org/jewish_action/03/2014/real-story-hebrew-pronunciation/
- Hebrew Alphabet Reading Lessons for Beginners https://www.youtube.com/watch?v=habVln7fqYY&index=9&list=PL96Ibcw54Sql4TU58P8oryMslRST51WMl
- Pronunciations of Hebrew http://www.jewishvirtuallibrary.org/jsource/Judaism/hebpronunciation.html#2
- Ancient Hebrew Research Center https://www.ancient-hebrew.org/index.html
- Victoria Hanna - Hebrew Vowels Demonstration https://www.youtube.com/watch? v=fGliOFFUkoQ&t=217s Please let me know if you find the sources interesting :-)
Struggling with this one, I didn't know תודה רבה was informal, I always though it can be used to say "thank you".