system
A chat on subreddit /r/me_irl
human
A big part of learning English is knowing where to put the emphasis in the word.

 I was listening to someone speaking and she said: man-DAT-ory instead of MAN-datory. 

And thats when it became clear how to spot non native speakers who speak english really well.  She could speak mandarin (she was chinese) but her English was as fluent as mine with very little accent. But some words just really strike you if they're pronounced with a different emphasis even if they're technically pronounced right.
gpt
Oh emphasis! I went to a work conference in East Africa, it was mostly Nigerians, I was the only non African. We all spoke English but were struggling to understand one another. Then it dawned on me, the Africans were placing the emphasis on other syllables. Once I got tuned in, it was ok. And I was mostly able to work out where to place the emphasis when I spoke, so it sounded ok to them. Emphasis. Where you put it, is so critical.