My Dad and I are learning Welsh ( me for the first time in nearly 50 years!) and he at the age of nearly 85!
On Radio Cymru and S4C he has heard 2 words for ‘because’ , which are ‘achos’ and ‘oherwydd’. Can someone please tell me which one it is correct to use in different contexts, or can they be used interchangeably? Many Thanks 
Both achos and oherwydd can largely be used interchangeably, with achos being predominant in spoken Welsh (if my own observations are correct).
(By the way, Welsh isn’t alone in having multiple ways to express roughly the same thing… for in English you could often use “for” instead of “because”, see what I did there?
Thanks so much for explaining this Hendrik- I will relay it to my Dad You are right that Welsh is by no means alone in using different words to mean the same thing.
It will be a mystery solved for Dad!
Oherwydd is slightly more formal, and furthermore means not only because but also because of (for which achos has to have an o in front of it - oherwydd y rhyfel = o achos y rhyfel = because of the war)
Thank you so much Gareth. It all makes sense, We have learnt some extra vocabulary along the way too 
even more formal and biblical if anything is oblegid