Tiny questions

Siamae Pawb,
The more I learn the more I realise there are a few words in Welsh that mean the same thing. Is it a matter of personal choice or do the words change with the context of the sentence ?
Could you look at the words I have come across and explain why one is used and not the other.
For … am Neu Ar gyfer
Promise… addo Neu Addewid
Make … creu Neu Gwneud
Tell me… adrodd I fi Neu Ddweud with I fi

Finally, I hear people on radio Cymru use the phrase, “ iawn iawn iawn”
What does that mean

Diolch yn fair.

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