I searched because I felt sure this must have been asked previously, but I found nothing:
For the future tense, when does one choose to use gwneud as the auxiliary and when to use bod?
I’m slightly clearer with the past tense, although not a lot. At least with that there are some parallels with the English past tense formed with “I did…(do something)…”
I think that as with the past, the future aspects of gwneud fit in with an action or mental action (intention, remembrance, etc), also for the polite “will you?”. I think that you can use the future forms of Bod (shall/will, etc) for situations and actions. So there is some room for overlap and choice.
The people I speak to most use gwneud all the time for actions.
“Ydy fy siaced yn y car?”
“Na i edrych”
Then wherever it’s “will be” in English they use bod.
“Fyddi di’n dod draw heno 'ma?”
Rough rule of thumb for you that we use 
Depends on what’s being asked. I tend to hear “gei di?” If you’re asking someone to do/get something. Or “nei di”
Ultra polite, as introduced in the old course I think, would be something like “Oes modd i ti/chi …”. I say ultra polite, just more polite maybe. 
Slight aside, (but it wouldn’t be the forum without one would it? 
) what makes a question polite? I’m ultrasensitive to the omission of the word please but also, I know that questions can be polite without a please and rude with a please, depending on tone. But are Welsh ears needing plis/osgdd as much as my English ears? With “ga i lwy?” is the plis noticeable when omitted?
(this conversation has come out of trying to teach a 3 year old to use please)
I’ve noticed that on road signs such as “Please drive carefully”, the Welsh “please” seems to be omitted and you have just “Gyrrwch yn ofalus”. Perhaps the “please” is implied by the use of “Gyrrwch” over “Gyrra”? Or maybe that’s simply because the signs are addressing multiple people! I don’t know. Struck me as odd, but I guess it’s perfectly normal.
As you say, gyrrwch addresses all drivers. Gyrra isn’t necessarily less polite. I wouldn’t use -wch over -a with a close friend or child and I wouldn’t be any less polite to them in English either.
Yes, implied I’d say as its known as the polite form. Other than that @AnthonyCusack yes, in my limited experience, I’d say other forms generally have the plis tagged on. I noticed Plis, Diolch and Sori in exactly the same measure as English. I’m not aware that the language itself has any bearing on when or where to use the pleasantries.
Another question (blame the online eisteddfod!): how does one say “There is something that I like…”?
Mae rhywbeth dw i’n hoffi…
Mae rhywbeth bo’ fi’n hoffi…
and is it the same if rhywbeth is replaced by something specific:
Mae siop bo’ fi’n hoffi…
It’s that subordinate clause bit that I find confusing.
(Also, how do you say something is called something? Mae e’n galw… would be “He is calling…” rather than “He is called…”, wouldn’t it?)
It is this option that gives you the ‘that I’ and therefore the linkage between the two halves of the sentence and the rhwybeth can be anything you want, as it were.
Yes it would
…if something has that standalone past tense in the English verb in a sentence which is otherwise present…it’s passive and you can get to the Welsh by switching the English so that the subject is ‘having’ the verb done to it - then translating to Welsh…
So…”He is called”…becomes…“He is having his calling”…
Mae fe’n cael ei (g)alw…
Hopefully that makes some sort of sense! 
Rich 
It makes much sense, thanks @rich. I’ve encountered that construct before, but am never quite sure how and when to use it.
there’s a soft mutation in there too because he’s masculine: cael ei alw
Was thinking about this, I’d probably say “ei enw ydy/yw…” rather than he is called, his name is
Yeah, you see this is the problem… it’s part of a longer sentence and I’m getting all tangled up in how to link the various bits together. I have used the “Ei enw ydy…” construction elsewhere, but it didn’t seem to fit properly for another bit. I can’t say too much because it’s for my entry for the online eisteddfod, and I don’t want to give myself away! Thanks for you help though.
In lesson 1 level 2 I have a tiny question. Adjectives after “yn” In “bod” sentences - which include “mae” sentences are supposed to undergo a soft mutation (eg mae’n ddiddorol) I just want to know I’m justifiably confused about the absence of this mutation in use of “llond” & “llond llaw” Llond looks like an adjective & I expected it to mutate to “lond” . Perhaps “llond llaw” is a nonverbal noun (not an adjective) in which case it would not mutate but then Id expect it not to be preceded by “yn” but it is. What am I missing?
The noun/adjective soft mutation after the “yn” following bod is what’s sometimes called a weak soft mutation. It doesn’t affect the letters Ll or Rh.
Yes…
Yes, frequency-wise it doesn’t seem to crop up that often in regular conversation in the present tense - although in the news people are stabbed, killed etc (cheerful ) - but again that tends to be in the storytelling ‘was’ tense or the past tense I suppose.
The same principle applies in the past tense eg to born or raised which are probably more common in regular speech aren’t they - where it becomes ‘had his birth’ or ‘had his raising’.
Rich
I’m trying to overcome my reluctance to read Welsh, but I’m reeeeeeally slow. Since it took me a few minutes to figure the word order 
I’ve just noticed another way to say somebody’s name:
Roedd un ohonyn nhw yn ferch hardd o’r enw Lisbeth
p.s. @rich, I’ve started with Colin Jones that you had mentioned as using language closer to spoken one than rather literary/academic. Simplified stories with audio tracks, like a 4 year old and I’m doing quite well! 