Please ask unrelated questions in separate threads.
/sʌb/. Devoicing is possible before a voiceless consonant.I'm also confused with the prefix sub (eg. substantially) səb or what?
You have to learn. There is no easy way around it. Listen to how native speakers pronounce the words that have "th" and try to remember. You can also look these pronunciations up in dictionaries.And then there's the th sound. I know that there are 2 th sounds; the voiceless and voiced, how do you know which to use?