Hi Shirley,
I think maybe your question is about 'T' pronunciation alone, instead of ton/tain/tan-->>N.
When you say the letter N (e.g. reciting the alphabet A-B-C-D-.....-N-....), you're not really making the N sound alone. You're saying 'en', or 'ain'. The N sound alone is just nnnnnnnnnnnnnnn (humming - it sounds like a small plane flying overhead, or a grass cutting machine 100m away).
So when you write CER-N, and BR-N, it is actually the sound of CER-AIN, and BR-AIN.
So, what is really changing?
The missing pronunciation is...the T!
This is something called T-glottalization and it happens in some situations in some English accents / dialects. And basically, I think the answer is...because English speaking usually evolves over time to become easier to say. Similar to how a river can change the shape of stone!
So, some British accents will say 'WA-ER' instead of 'WATER', because the flow of speaking is smoother if they just stop the air there, instead of making a clear T pronunciation.
That is the same reason in a lot of people in North America will pronounce 'water' as 'wader', and 'competitive' as 'compedidive', instead of saying true T sounds in the middle of the word. It is smoother and faster to make a D sound.
But not everyone in Britain skips middle-word-Ts, and not everyone in North America changes middle-word-Ts to D's.
Here is a video discussing it!
https://www.youtube.com/watch?v=TtcLrYwyfPs
And if you really want to dive into the details, the wikipedia article (there might be similar articles in your native language also!) is probably a good starting point.
https://en.wikipedia.org/wiki/T-glottalization
Another similar regional difference in accents is when an R-sound suddenly appears!
https://en.wikipedia.org/wiki/Linking_and_intrusive_R
And 'connected speech' is a good topic to search to learn about how connecting words can change the sounds!
I hope that helps! If you have any other questions, let me know.