I have difficulty reading this sentence: "I helped them"
ipa: ˈa͡ɪ hˈɛlpt ðˈɛm
I don't know how to position my tongue when pronouncing the consecutive sounds /t/ and /ð/.
The Knick (2014) - S02E01 Ten Knots clip with quote and I helped them. Yarn is the best search for video clips by quote. Find the exact moment in a TV show, movie, or music video you want to share. Easily move forward or backward to get to the perfect clip.
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In American English (at least), /t/ sound (before /th/) is usually unaspirated:
"sit there" : ([2:22] sit there)
Or "at the" : ([1:24] at the)("... slide the tongue forward ...")
As a teacher, I'm two minds as to what advice to give you. You face two choices:
1) Try to imitate the linking between the two words that a native speaker would likely use.
2) Don't bother. Just pronounce the final '-ed' as /t/ and move on to the following sound.
Given the scant evidence of your speech that I can hear from the audio file you posted, I lean towards recommending the second option. It's more important that you be understood than you sound like a native speaker.
And more important than the linking is that the vowel you're producing in the word 'them' is too strong. Unless you have a particular reason to stress that word, the vowel should be in a weak form.
For clarity, 'unaspirated' means you make the closure but not the plosion - you move straight from the closure of the /t/ to the fricative. A similar thing happens
in some words, but with the fricative unvoiced: e.g. 'eighth' ( /eɪtθ/)