Can someone explain to me the missing letters when you see 'r or 'n.
The missing letter in the cases of 'r and 'n is most likely the letter y. These forms of yr and yn are used when they appear after a vowel.
Dw i’n hoffi coffi - I like coffee (not Dw i yn …)
Mae’r gath yn hoffi llaeth - The cat likes milk (not Mae yr gath…)
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And in addition, 'n can sometimes stand for ein our
Croeso i’n tŷ ni
Welcome to our house
But the yn that @Hendrik mentions is of course much much more common and likely 
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'di is for wedi
The list of abreviations in S. Wales Welsh and also Englidh for that matter is a fairly long one. As the plan is to shorten everything. 
This thrrad could run on and on 
Ah sorry just r and n.
Ok like R’o’n l’n?
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