I checked the pronunciation of posthumous in dictionary which reads /ˈpɑːstʃəməs/, and then wondered if it is /t/ and /ʃ/ separately as in tip /tɪp/ and shoe /ʃuː/ or it reads /tʃ/ as in chair /tʃer/. I understand that the sounds are very similar so possibly it doesn't make any difference either way. Is that the same for native speakers, too? I mean is both versions of saying the word true and it doesn't matter? If not, is there a rule or a trick to help me read the pronunciation of the word and learn it correctly?
There are cases where /tʃ/ is pronounced as two separate sounds, /t/ followed by /ʃ/. However, this consonant sequence is very rare, it can only occur when the consonants are separated into different syllables, and it only occurs in compound words.
When it's necessary to disambiguate, the sequence of two distinct sounds may be written as /t.ʃ/, and the single affricate sound of "chair" may be written as /t͡ʃ/. Posthumous is pronounced with the affricate phoneme /t͡ʃ/ (or, as Curtis White mentions, certain speakers may use /tj/).
Example words with /t.ʃ/: hotshot, nutshell, batshit, potshot, sweatshirt, sweatshop. Note that they are all compound words spelled with "t" followed by "sh". I got them from a Onelook Dictionary Search of the sequence "tsh". There are no words where "t" or "th" represents /t.ʃ/.
Phonetically, /t͡ʃ/ and /t.ʃ/ differ on average in various ways. The ones I can think of are:
- the period of friction (the [ʃ] sound) is longer on average in /t.ʃ/ than in /t͡ʃ/.
- the /t/ in /t.ʃ/ may be subject to glottal replacement, resulting in [ʔʃ]
It differs by dialect, but some dialects pronounce it as:
/tʆ/ like chair, so /ˈpɑːstʆəməs/
However, other dialects, including mine, pronounce it as /t/ and /(h)ju/, so /ˈpɑːst(h)jəməs/
Others still may say: /postˈhjuməs/
I would pick whichever one who feel most comfortable with and be consistent. You should be understood by most people whichever one you pick.