"the"- pronunciation
strong form /ðiː/ and weak form /ðə/ both can be heard in one recording.
why and what's the rule?

Question

Answer

/ðiː/ before words begin with a vowel sound:
the American people
the onions
the egg
/ðə/ before other words
the man
the girls

Answer

Before a vowel sound, it's far easier to say /ðiː/ than /ðə/.  Try it. ^^

Does that match with your recording as well? :)

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It doesn't sound odd to just use whichever you want.  They are the same.

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I tend only to say /ðiː/ for emphasis, and /ðə/ for all other cases, but that might be a British thing.