When to use do vs gu

[Ionatan]

I am a bit confused with when to use do vs gu.

The dictionary says:

do prep 1. for, 2. to, unto
gu prep 1. to (a place or point in time), 2. till, until, unto, 3. for

So they both can mean 'to' and 'for' but clearly are not interchangeable (or are they?) so there is some nuance. For example we would say:

Tha ma a' dol don bhùth (I am going to the shop)

...but...

Tha e cairteal gu trì (It is a quarter to three)

If gu means 'to a place' (see above) and also 'unto' (though so does do), why do we not say Tha me a' dol gun bhùth (or is that permissable)?

The best I can figure it out is that gu means something more like 'up to' as in 'up to now' 'up to here'. It would seem that gu has more to do with time and do has more to do with space (it's a shame I can't ask Stephen Hawking about it ). Which would explain why we say gu bràth (forever == until judgement day) and not do bràth perhaps?

Is that right?

PS - I realise that neither is used in the infinitive of a verb because, unlike Beurla the infinitive is just one word (like many/most languages e.g. French etc).

[faoileag]

Basic principle:
You can never tr*nsl*t* prepositions directly from one language to another. (Or much else, but particularly preps.)

Think of gu, as you have worked out, as approx up to, as far as, until, or as a waymarker in a journey. Whether you enter the place you reach is immaterial.
Very often paired with bho (explicitly or implicitly), in "from... to...":
Feumaidh tu dràibheadh bho Ghlaschu gu Dùn Èideann, agus an dèidh sin thairis air an drochaid.
Bha mi ag obair bho Dhiluain gu Diardaoin.
Bha mi ag obair gu sia uairean agus an dèidh sin chaidh mi dhachaigh.

Do
To / into - when you want to make it clear you went right into the place. (Not used, as far as I can think at the moment, for time.)
Chaidh mi dhan bhaile / dhan eaglais /dhan Ghearasdan / do chèilidh / do chuirm-chiùil.
With place names without an article it tends to be abbreviated to "a" (no accent, and it lenites), and usually gets a dh' before vowels.
Chaidh mi a Ghlaschu / a dh'Inbhir Nis.

Do
can also mean for, destined for, as a gift for etc. (It's still roughly "directional" - a practical or metaphorical transfer to someone)
Seo leabhar inntinneach dhut.
A bheil beagan airgid agad dhomh?
Cheannaich mi tiodhlac do mo mhàthair.
Tha sin doirbh dhomh.

Just to add to the learning load, but a useful strategy, there are lots of expressions and verbs which "take" particular prepositions, and you just have to familiarise yourself with them, e.g.
cuir gu - send to. Chuir e post-d / litir / pacaid gu Calum.
cuir fòn gu - phone s.o. Chuir mi fòn gu Calum.

bruidhinn ri - speak to
èist ri - listen to

's urrainn dhomh - I can, am able to
bu chòir dhomh - I should , I ought to
thoir dhomh do làmh! Give (to) me your hand

(You can happily keep learning forever! It's fun! )

[Ionatan]

Mòran taing. That is very helpful.

I am sure I will get these wrong for some time - especially when you add ri into the mix.

[faoileag]

DON'T add ri to the "to" mix!!!!!!!!!!!!

It has nothing to do with motion or direction. It covers an interface situation - you share a surface / space with someone, something, an activity.
Learn it in phrases, or together with the verbs that usually "take" it. Stay clear of equating it to an English preposition.

Good useful ones to learn now:

Dè tha thu ris? What are you up to? What are you doing? (Answer doesn't need to have ri)

bruidhinn ri, èisteachd ri
còmhla ri - (together) with (a person)
coltach ri - similar to, like

ris a' bhalla - leaning against the wall

Tha iad ris an iasgach (etc) - they're at the fishing , busy with the fishing, working at the fishing (or any ongoing activity)

[Ionatan]

<extracts 'ri' from the mix>

I have just learnt còmhla ri in Can Seo.

I think seeing it in phrases like bruidhinn ri and èisteachd ri is what caused my confusion there. However, your description of it being an 'interface situation' is very helpful and that becomes clearer when those two phrases are compared to ris a' bhalla. So it looks like ri also has nothing to do with time (which would seem to be more 'gu' territory).

I wouldn't like to say it's all completely clear now but my grasp is a certainly less tenuous.

[GunChleoc]

This is a good read about ri: http://www.akerbeltz.org/index.php?title=Ri