Diolch yn fawr, @netmouse! Very helpful! I see that the object pronouns determine the mutations, not the “wnaethon”.
Actually, not quite.
In Netmouse’s examples, sentences 1, 3 and 5, i.e. “wnaethon ni fenthyg y llyfrau”, for example, it is the subject, “ni” which is causing the mutation, and it is always a soft mutation in cases like this. (See Gareth King’s grammar book when you get a chance to).
In the sentences 2, 4, and 6 (i.e. “Wnaethon ni eu benthyg nhw.” etc) something slightly different is going on.
In those cases, it can’t be the subject (“ni”) causing the mutation because it’s not next to the word being mutated. So in that case, it is, as you say, the object pronoun causing the mutation, but it depends on the gender, and whether it’s singular or plural.
There is a bit more to this - these object pronouns are very similar to (may be the same thing as ) possessive pronouns,and in those cases, different types of mutation can happen, depending on gender. But I don’t remember the rules 100%, and would have to look in a book, or go back through some lessons, so I better leave it there. 
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Modern Welsh, paragraph 14, p. 21
My understanding of Gareth’s rule (observation?) is that a verb’s complement (usually the object of the verb, but not always) is soft mutated where possible, whether the subject of the verb is explicit or not. in @netmouse 's examples, “fenthyg y llyfrau”, “eu benthyg nhw”, etc are all complements of wnaethon, and ni is the subject. Soft mutation occurs where possible “fenthyg”, and does not occur otherwise “eu benthyg”.
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Thanks for the clarification, @mikeellwood. In my mind, I just think that when we use the “wnes i”, “wnaethon ni”, etc. forms, we soft-mutate the following verb, as we learned in the lessons. I never thought about the subject being the thing that was causing the mutation. I do know some of the object pronoun mutations, because I’ve done a few lessons with those, but not all of them yet.
@louis Thank you for your explanation. I have Colloquial Welsh and the Modern Welsh Dictionary; I don’t have Modern Welsh (yet). I think I follow what you are saying, but I’m not entirely sure about what would constitute “possible”. Apparently I need to learn a lot more about Welsh grammar.
I’m pushing my limits with all this, but find myself wanting possessives or object pronouns as I go about my daily practice. So I’m a bad girl, kludging together what I’ve learned in lessons so far, what I pick up from dipping into my grammar book and dictionary, and what I pick up from other listening/reading. I am finally reaching a point where I will have more time and energy to get back into lessons. Can’t wait to start learning “properly” again!
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This is of course an entirely optional activity! I learnt them almost exclusively from course 2, and only recently got round to googling the exact spellings. May possibly put off reading Gareth King for ever… (Sorry @garethrking !)
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The great majority of grammar books (ie, pretty much everybody except Gareth king) seem to regard thus as the soft mutation of the direct object of a declined verb. That’s pretty much the opinion of the tiny percentage of Welsh speakers I know who think about such matters as well.
Not saying Gareth King is wrong, (or that you can be right or wrong on these matters!), just that there are other people writing on Welsh grammar (some of whom are pretty well respected) and that Gareth King seems to be in a minority on this one. And nothing wrong with that.
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Ooh dear, I haven’t got the slightest clue what you are all arguing about!
But presuming my original examples were correct and make logical sense, probably the most appropriate mantra is “Don’t worry about it!” 
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Quite right!
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Ah, sorry - beginning with a consonant that can soft mutate
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That’s what I understand as well. If my understanding of what Gareth writes is correct, this is one of the patterns, he has generalised the patterns for syntactic soft mutation to include cases other than “direct object of declined verb” - but I may be wrong on that
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I think after reading the whole discussion here, this is going to be my best option at the moment!!! It’s all starting to get too far above my head…but I asked for it 
Always good advice
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This generalisation (ie it being part of a larger syntactic VSoO soft mutation group rather than simply sm of the direct object of a declined verb) is what other people don’t do (as far as I can see.) Gareth King seems to imply the subject causes contact soft mutation. The great majority of grammar books discussing soft mutation don’t do this (and a lot of people have worked on them for quite some time), being more of a disparate collection of things. One being sm of direct object etc. And this also fits in with the views of the few Welsh speakers I know inclined to consider and talk about such things, but that means little. This is not because such suggestions haven’t occurred to them. If Gareth Kings views and rules on the matter are better for anyone, or work for anyone, or make them easier for learners to remember, that is great. In fact, it is terrific. But it is worth pointing out this one is a minority view. Anyone interested in it, best to do some reading of different writers on the subject and make your choice! (addressed to anyone else interested - I’m sure Louis has done that already!)in
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The Syntax of Welsh (e.g. p. 237) makes a distinction between a direct object (basically a noun phrase) and a verb phrase, and points out that they are both soft mutated. One example they give is “dechreuodd Elen yrru bws” - “Elen started to drive a bus”. The contention is that “yrru bws” is not a direct object, because it is not a nominal phrase. If you agree with that, then a generalisation is a good idea, if you don’t, then it is also good
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Verb nouns also undergo soft mutation in such circumstances, that’s a well accepted thing- can’t see the problem, as it were- but that is not saying the subject causes contact soft mutation, as Gareth King says in modern Welsh in the paragraph and page you quote above. (" [SUBJECT] o")
If you are impressed by that rule, good for you. If it helps you remember stuff, eve better. I don’t have good enough Welsh to make a decision one way or another, and just going by what I have read and heard from other people, it seems to be a minority view, for what that is worth.
I don’t really think rules of grammar can be “discovered” - they exist in the language because they exist to a certain conscious degree in our heads, this is why they are used, extended, incorrectly applied to irregular verbs etc. If people haven’t come up with that rule in such a length of time, I would say it is probably because it isn’t there. And if Welsh speakers themselves look on soft mutation as being inspired by a long, disparate list of things, and the history behind the reasons for soft mutation of those things is different, I would say there is no need to try to shoehorn the rules into larger, easier groups as it were.
But there are certainly different ways of analysing things, However, as I say, if it helps people to remember, great! Just remember that if I understand correctly, by that analysis the subject is sometimes not obvious, or indeed there at all.
[edit - as an example, I have a friend (a natural Welsh speaker, and translator as it happens, who hates grammar with a vengeance, but one of the few rules he makes sure to follow himself is soft mutating the direct object of a declined verb- not soft mutating after a subject, as it were. How aural Welsh speakers regard these things is important, not just theoretical possibilities of division. Just one very minor example.)
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I am fleeing away!! (Not forever, just from this discussion!). It seems to stem from a tiny question which does not have a quick answer!!! Would it help anyone else if the Grammar buffs started a thread on complex grammar questions??? 
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I think it might in fact be helpful if advanced learners had a thread to discuss complex grammar questions, and Welsh syntax (that makes me cry) and etymology! I would sometimes read it, feeling like an illiterate peasant girl peeping at well-educated ladies and gentlemen through a key-hole:)
If that means ‘how words derived’, I am actually interested in that, so maybe it needs a separate thread from syntax and deep, dark grammar!!! 
Yeah, this!
I’m interested in it too, and would very much like to research it, but unfortunately the day only has a limited number of hours, so I chose folklore and mythology instead:)
By the way, there is a thread on Welsh etymology here, I’m sure. I remember putting the link to it into my list of helpful threads on this forum.
Update
It’s here:) Welsh Etymology (Word Origins)
Oh, incidentally, though I say there are other writers on grammar, I find it difficult to think of anyone who is a better writer than Gareth King on such stuff for learners! (IMHO, ynfmi)
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To @stella Diolch!!!
to @owainlurch I totally agree, not that I have much experience of other writers!!
p.s. I am putting a post in the comments on SSiW thread, because I think it fits there… contrasting GCSE results with SSiW!!