th followed by an s sound [duplicate]

What is the correct way to pronounce such complicated combination of sounds when not pausing for breath? As an example, how would one pronounce something like "The Eighteenth century"?

In normal speech, consonant clusters are generally simplified, because, as you point out, they're complicated and difficult to pronounce.

For instance, the pronunciation of the fractional plural sixths, as in five-sixths '5/6', is sposta be /sɪksθs/, ending in a godawful cluster of four voiceless (i.e, whispered) consonants. But nobody ever says /sɪksθs/. The /θ/ between the /s/s is simply deleted, leaving a long /s/ at the end to mark the difference between '5/6' /fayv.sɪkss/ and '5, 6' /'fayv.'sɪks/.

This is why there are contractions; the faster you talk, the more stuff you drop, not only because of pronunciation difficulties like this, but also because English is a stress-timed language, which means that there is usually the same amount of time spent between each major stress in a sentence, no matter how many unstressed syllables there are between them. In fast speech, multiple unstressed syllables are usually only a lick and a promise, if they're there at all.

In particular, the Eighteenth Century is pronounced /ðiyetinsɛnʃri/. Nobody ever notices because that's what's expected. One more reason why the article is there -- the Eighteenth Century is not 18 centuries, so we can tell the difference if they're pronounced the same.

To Repeat: English spelling does not represent English pronunciation.

There is no change. Those both sound just like if the two words were said in isolation.

I don’t know why you think there might be a change, nor why you consider that a “complicated combination of sounds”. Native speakers would not.

On the other hand, one common pronunciation of months suppresses the th, so maybe that is what you have been hearing. Children are known to sometimes say moss for moths, but they eventually correct this lest they be perceived as having a speech defect.

A following sibilant does not neutralize an earlier fricative, at least in careful speech. In fact, phonemic voicing is maintained in noun–verb or in possessive–plural pairs like:

• cloths, clothes
• breaths, breathes
• mouths, mouthes
• wolf’s, wolves
• calf’s, calves
• house’s, houses
• moth’s, moths

In the first word of each pair above, that word ends in two unvoiced sounds, whereas in the second word of each pair, it ends in two voiced sounds. Sometimes this is (somewhat) reflected in the spelling, but often it is not.

As I mentioned in comments, you should practice correctly saying fifths, sixths, twelfths until you have it down pat. Then once that’s done, you can then proceed to

The sixth sick sheik’s sixth sheep’s sick.

Which is surely one of the most pleasurable phrases in the English language. :)

Ok, I’m just kidding. These can be hard for anybody. But in careful speech, they can certainly be done. Very, very careful speech, perhaps, but not impossible.