Today I did Course 2 Lesson 22 North. At about time 11’ 15" (on my MP3 player) I heard the construction “achos i mi anghofio” as a translation of “because I forgot”. I was expecting “achos wnes i anghofio”. I don’t remember this construction actually being introduced by Aran. I suppose that “achos i mi anghofio” is equivalent to “because of my forgetting”. Is this the same construction as “cyn i mi”?
Comments, or is it insignificant?
Diolch.
Great question! Above was my gut reaction. I’m mainly commenting to find out the answer 
Mine was ‘achos dwi wedi anghofio’. Posting for same reason as yours!
Yup, this would have been completely fine - just throwing in a few variations to help prepare you for decoding real speech…
That’s what I like about SSiW. Keeps us in the real world.
Achos i mi anghofio sounds more Welsh (I dare to suggest) than Achos wnes i anghofio, which sounds English to the point of awkwardness I would say - though others may disagree.
The root issue here is that achos is a conjunction, and so in Welsh we would expect a subordinate (i.e. ‘that…’) clause after it. Same is true with Pam…?, incidentally.
So I would always recommend (first of all)
Achos bod nhw’n dod
Because they’re coming
rather than Achos maen nhw’n dod, which to me at least sounds not quite right.
Similarly
Pam fod e fan’ma?
Why is he here?
rather than Pam mae e fan’ma - dodgy to my ears at least.
The next thing then to remember is that the little preposition i does have a ‘that…’ meaning’ - this is the real reason that we say (as @Baruch astutely spotted) cyn i mi fynd, and lots of other time conjunctions too. Note also that many dialects actually use cyn bod/fod….instead - just making the ‘that…’ even more apparent.
And when there is a sense of past tense, i comes even more into its own as ‘that…’, which is why
Achos i mi anghofio
Because I forgot
is such perfect Welsh!
I hope you’re all taking notes - there’ll be a short test on this next period. 
So, would “achos bod mi anghofio” be more “because I am forgetting” and “achos i mi anghofio” be “becasue I forgot”?
This feels subjunctive to me, btw…
What about ‘achos anghofies i’?
Essentially, yes.
Achos mod i’n anghofio (slightly more formal)
Achos bo fi’n anghofio
Both of these mean ‘Because I forget’ or ‘Because I’m forgetting’, and a few other variations as well probably.
And Achos i mi anghofio definitely has (to my ear at least) a past-tense connotation - ‘Because I forgot’, or indeed ‘Because I had forgotten’.
Isn’t this fun?
That way lies madness, @Bobi - stop using the S-word, please.
Dare I also point those feeling bold today to §491, §503 and §504 in the Modern Welsh for added fun on these matters?
Chances are that those of us involved in this thread will remember this detail far better than had we really been taking notes in a formal classroom environment! I certainly will. Diolch yn fawr.
For anyone worried about "getting it wrong:
I’m not entirely confortable with achos bod, because I tend to use oherwydd bod (oherwydd and achos in speech have an identical meaning). On the other hand, achos mae sounds perfectly acceptable to my ear. I’ll be absolutely frank and say I have no idea whether that’s because it’s widely used or whether it’s because I spend a lot of time with second language speakers, and have never really noticed that it’s not 100% correct.
I am almost certain that bod here is not what is used in Llandysul at least, and definitely seems uncomfortable to me. IE, I would definitely say “pam mae fe fanna?” as a question. In a statement, I would tend to use bod - " dwi ddim yn gwybod pam bod e fanna."
I’m not saying this to disagree with @garethrking - I’m a Welsh speaking ex-engineer, not a linguist or academic - but rather to reassure you as second language speakers a) that there’s no need to worry about this level of detail until you’re ready for it and b) that you will get away unnoticed with far worse crimes against grammar than either of these grammatical details!
We’re all learners here (including me), and I for one will be listening out when I hear this pattern form other Welsh speakers!
Diolch yn fawr Gareth as ever.
Some of us can’t get enough grammar Gareth (even if we indulge our fancy behind closed doors… 
).
Just to help with that:
If I were you I’d leave the subjunctive out of it?
How would English (or Welsh) sound if it wasn’t for the subjunctive 
?
Long live the subjunctive!
That is also very interesting, and just goes to show that usages differ across the country. I learnt Welsh from gog girls, and that may well be the difference here.
The good news for learners is that you can clearly get away with either construction, can’t you?
Doed a ddelo!
Again @Karla, I don’t like the sound of that myself - sounds very very translated-from-English. But I’m guessing in some parts of Wales they wouldn’t bat an eyelid at that.
And you’d certainly be understood. The situation is NOT as follows:
Achos i mi anghofio - relaxed native-speaker understands: ‘Because I forgot’
Achos anghofies i - bewildered native-speaker understands: ‘How much are these Sumatran bananas?’