I thought mutations had to do with something preceding the mutating words. But maybe not!
I keep on finding i mewn and i fewn and I don’t understand why one or the other.
And then why sometimes it’s tu mewn?
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Possibly, they just aren’t applying the mutation every time in real life.
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According to Christine Jones and her book, there are 49 rules for mutations. I tried to find the list online, but couldn’t. Sorry, Gisela-Albertini. https://www.amazon.com/Welsh-Grammar-Really-Teach-Yourself/dp/1444189638/ref=sr_1_fkmrnull_1?keywords=welsh+grammar+you+really+need+to+know&qid=1554043025&s=gateway&sr=8-1-fkmrnull
I’m not an expert with mutations, far from it, but going from ‘m’ to ‘f’ is a soft mutation and Christine Jones has 31 rules. I don’t know which rules apply, but I suspect several. 
I can answer why tu mewn. Tu mewn is used to connote going “inside” something, like a house or building. Dw i eisiau mynd tu mewn, achos dw i’n oer (I want to go inside, because I’m cold.) Whereas, ‘mewn’ is ‘in’ or ‘in a.’ For example, Dewch i mewn (come in.)
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@garethrking’s dictionary has a good section in the front summarizing the rules for mutation. Whenever I have a question I refer to that, even before the grammar book. You can tell because the pages are coming loose!
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They are listed simply as alternatives in The Modern W Grammar [460] - no difference in meaning- with i fewn mentioned as being less common. So yes it is unusual - I guess as a set phrase it’s one of those things that it what it is…(?)
Rich 
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Yes - tu is a lesser-known word for side,(which is usually ochor of course) - and there’s a whole set of tu- placewords indicating position:
tu fas/mas
tu fewn/mewn
tu ôl
tu draw
tu hwnt
tu cefn
and whatever you’re having yourself…
And if there’s a following noun, you link it with i:
tu cefn i’r bar behind the bar
tu draw i’r afon on the other side of the river
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That is a bit intimidating, I think, and not really true as RULES.
All mutations are either contact mutations (caused by a preceding word) or grammatical mutations. NM has contact mutations only.
AM and SM have both (though actually AM only has the one grammatical instance).
So you simply learn the words that cause contact mutation (there are only two for NM, and six for AM - admittedly quite a lot more for SM, but still perfectly manageable, about 30 + a few common prefixes), and then you try and keep in mind the five grammatical instances for SM:
1 after the subject of the sentence
2 adverbs saying ‘time when’
3 vocative - when addressing someone (but rare actually, because personal names don’t mutate)
4 infected verbs (i.e. with endings - the Preterite I and Future I)
5 after an intrusive word in the normal sentence (not awfully common really)
And the single grammatical instance for AM:
1 negative inflected verbs (often not done in many dialects of Welsh anyway)
And that’s it!
And remember that 90%+ of instances of mutation in the living language are SM - so master that first.
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I spent several seconds seriously considering whether this was a specialist term of Celtic linguistics I hadn’t come across before (cf. ultimate i-affection ayyb), before eventually deciding it was a typo…
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I believe you can get something for this from the chemist these days.
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Oops to both of you! 
It’s been a long day…
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Thank you for clarifying about them NOT being rules, and I’ve always felt the number was intimidating but there are so many members who say, don’t worry about, that I don’t.
Thank you for your brief summary!
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This time I had remembered to check it out
(along with mewn, i mewn, tu and inside) before asking. The only ‘rule’ that seemed to apply (for what I could understand) was being preceded by i. But they both were in the examples I had found so I was still confused 
Yes - there are only 49 (or whatever) ‘rules’ if things like ‘SM after gan’ and ‘SM after neu’ count each as a separate ‘rule’, I suppose. Not helpful, I think - just hand out a list of all the words that cause SM (30-odd) and order everyone to learn them by 9.00 Monday morning or else. Simps!!
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I think “i” does account for more than it’s fair share of soft mutations.
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I remember being in a cafe in mid Wales (can’t remember the name of the place off the top of my head, lots of waterfalls and bridges) and after ordering beans on toast “heb menyn” among other things the lady repeated the order back to me and with a little deliberate glance up and a smirk said “heb fenyn”. I said “those darn soft mutations” and a younger girl behind the counter realed off this list … ‘am’, ‘ar’, ‘at’, dros’, ‘drwy’, ‘dan’, wrth’, ‘o’, ‘i’, ‘heb’, ‘tan’, ‘gan’ … without thinking saying that it was drilled into them at school from an early age. I really should learn it, and I will one day … one day.
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No time like the present, @gruntius - and it will improve your Welsh no end, as well as making you feel wonderful!
C’mon - 30-odd little words…you can do it!! 
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I swear I’m not trying to find an excuse to avoid studying the list (that, by the way, is also on the Modern Welsh dictionary, so I have it!). 
But along with:
ei laswellt, ei gyflog
dy ddisgwyl, dy galon
mor garedig
I’ve also found:
dyma pam, dyma’r barn
dyna beth
dy chwaer
What happened?
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Excellent question, Gisella - here’s what happened:
dyma pam - pam is one of a small number of words that never mutate (mae is another)
dyma’r barn - no mutation of barn because 'r is in the way - the contact mutations only work when nothing gets in the way
dyna beth - beth is already mutated anyway from peth, and you can’t double mutate
dy chwaer - ch is a different letter from c, and never mutates
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Contact mutation…sounds positively awful…
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