u/HyderNidPryder

amlwg - obvious, evident, apparent, prominent

arhosfa[n] fysiau (b) ll. arosfannau bysiau - bus stop

egwyl (b) ll. egwylion - break, respite, interval

sefydlu (sefydl-) - to make stable, to found, to establish

arweinlyfr (g) ll. arweinlyfrau - guidebook, guide

newynog - starving, famished, hungry

bws gwennol (g) ll. bysiau gwennol - shuttle bus

rheithgor (g) ll. rheithgorau - jury

cyffroi (cyffro-) - to stir, to agitate, to excite

dyddodi (dyddod-) - to deposit (layer of sediment, fat), to precipitate

neges destun (b) ll. negeseuon testun - text message

cerdded (cerdd-) - to walk

cwd (g) ll. cydau - bag, pouch, purse

powlen (b) ll. powlenni - bowl

llwyfan (b, g) ll. llwyfannau - stage (for performance)

anghysondeb (g) ll. anghysondebau - inconsistency

strelio (streli-) - to rinse

segur - idle

lleoli (lleol-) - to locate

orgraff (b) ll. orgraffau - orthography

o'r crud i'r bedd - from the cradle to the grave

rhywun (g) - someone

alaw (b) ll. alawon - melody, tune; lily

modd (g) ll. moddion - means, way, manner, method; moddion - medicine

golau (ans.) - light (in colour)

deintyddol - dental (relating to dentistry)

iaith arwyddion America (b) - American sign language

gwanc (g, b) - greed, craving

gyr (g) ll. gyrroedd - herd, drove, flock

deddfwrfa (b) ll. deddfwrfaoedd, deddfwrfeydd - legislature, legislative body

suddo (sudd-) - to sink

cydymdeimlad (g) ll. cydymdeimladau - sympathy, condolence, commiseration

aros yn stond - to stand still, to remain stationary

perigl (g) ll. peryglon - danger, risk

pensiynwr (g) ll. pensiynwyr - pensioner

cynghrair (g) ll. cynghreiriau - league, alliance

cyfansawdd (ans.) - composite, compound

talar (b) ll. talarau - headland, unploughed field margin

Crynwr (g) ll. Crynwyr - Quaker

cysefin - first, primary, original, primitive

oren - orange (orange-coloured)

dwgyd - (=dwyn) to steal (colloquial De Cymru)

llafn (g) ll. llafnau - blade (of knife etc.); young man, stripling

ar eich liwt eich hun[ain] - on your own (independently)

pob lwc! - good luck!

deuparth (g) - two portions (of three), two thirds; double portion

beirniadol - critical, criticising

canolog - central

cwest (g) ll. cwestau - inquest

lled-ddargludol (ans.) - semiconductor, semiconducting

cysgod (g) ll. cysgodion - shadow, shade; shelter

go brin - hardly, scarcely

hoe (b) ll. hoeau - pause, break

ynteu - or; then; in that case

ymennydd (g) ll. ymenyddiau - brain

tarian (b) ll. tarianau - shield

ysglyfaethus - predatory

hwylustod (g) - ease, facility

cyfoed (g) ll. cyfoedion - one of the same age, peer

bryngaer (b) ll. bryngaerau, bryngaerydd - hill fort

gwadd (b) ll. gwaddod - mole (animal) (De Cymru)

twrch daear (g) ll. tyrchod daear - mole (animal) (Gogledd Cymru)

unffordd - one-way

eto - again, yet, still

syth - straight, direct

pall (g) - failure, defect, lapse, cessation, end, limit

cymeradwyo (cymeradwy-) - to approve, to accept, to applaud

cydsyniad (g) - consent, agreement

pwmp gwres (g) ll. pympiau gwres - heat pump

dynladdiad (g) ll. dynladdiadau - manslaughter, homicide

Edit: I meant before verbnouns!

As well as being used for "in a place" mewn / yn are also used with verbs / verbnouns.

ymddiried mewn / yn - to trust in

ymddiddori yn / mewn - to take an interest in

ennyn diddordeb mewn / yn - to kindle interest in

cydio / gafael mewn / yn - to grasp something

Mewn is used before indefinite things and yn is used before definite things.

See here for examples.

ymddiried ynddi hi - to trust her

gafael yn dy law - take your hand

ymddiddori mewn llyfrau - to be interested in books

ymddiddori mewn llên gwerin - to be interested in folk literature

ymddiddori yn hanes Cymru - to be interested in the history of Wales

A verbnoun can also be used. Usually this is preceded by mewn.

ymddiddori mewn darllen - to be interested in reading

ymddiddori mewn canu - to be interested in singing

Now to my area of uncertainty:

Is a noun phrase starting with a verbnoun always indefinite? Does it always use mewn regardless?

ymddiddori mewn ymweld â cestyll - to take an interest in visiting castles [this is fine]

But is it

ymddiddori yn ywmeld â'r castell ? [yn rather than mewn?]

Is it a visit? or visiting a specific castle? Is this definite?

What about dw i'n ymddiddori yn prynu ei gar? - I'm interested in buying his car. [Is it yn or mewn?]

This case seems very specific - more the buying of his car than a buying of some unspecific car. Does it use use yn?

ymddiddori mewn darllen llyfrau - to be interested in reading books [yn or mewn? In English you could say "the reading of books"]

ymddiddori mewn darllen llyfr - to be interested in reading a book

ymddiddori yn darllen y llyfr - to be interested in reading the book [yn or mewn?]

ymddiddori (ymddiddor-) (mewn / yn) - to be interested (in), to take an interest (in)

clun (b) ll. cluniau - hip, thigh

mislif (g) - (menstrual) period, menstruation

gofid (g) ll. gofidiau - trouble, sorrow, hardship

rhestr (b) ll. rhestrau - list, row

dylunio (dyluni-) - to design

canolfan gwaith (b) ll. canolfannau gwaith - job centre

aeron - berries; u. aeronen (b) - berry

twymdra (g) - warmth, hotness, heat

Treth ar Werth (b) (TAW) - VAT (Value Added Tax)

breuddwyd (b, g) ll. breuddwydion - dream

tun (g) ll. tuniau - tin, can

treiffl (g) ll. treifflau - trifle (pudding)

diffoddwr tân (g) ll. diffoddwyr tân - firefighter, fireman; fire extinguisher

ei gilydd / ein gilydd / eich gilydd [never *eu gilydd*] - together, each other

cawg (g) ll. cawgau - bowl, cup, basin, crucible

caledi (g) - hardship, adversity

melysydd (g) ll. melysyddion - sweetener

crachach - snobs, conceited upstarts

uwchraddiad (g) ll. uwchraddiadau - upgrade

beth? - what?

noswaith (b) ll. nosweithiau - evening

Tyrd / Ty'd (ll. dewch / dowch) 'ma! - Come here! (Gogledd Cymru)

atgoffa (atgoff-) - to remind, to recollect

ymddangos (ymddangos-) - to appear

nwydd (g) ll. nwyddau - article of commerce; nwyddau - goods, merchandise

dirdynnol - excruciating, tormenting, harrowing

mewnforion - imports

amherffeithrwydd (g) - imperfection

cyfryngiad (g) ll. cyfryngiadau - interjection, interposition; intercession, mediation; arbitrage

nofio (nofi-) - to swim

rhosyn (g) ll. rhosynnau, rhosod - rose

rasio (rasi-) - to race

yn wreiddiol - originally

ochr (b) ll. ochrau - side

gwynegu (gwyneg-) - to ache

pererindod (b, g) ll. pererindodau - pilgrimage

fferyllydd (g) ll. fferyllyddion - pharmacist

ysgythru (ysgythr-) - to engrave, to etch

cymhellol - urging, stimulating, driving. persuasive, compelling

hen daid (g) ll. hen deidiau - great-grandfather (Gogledd Cymru)

y rhai melyn - the yellow ones

medd - he/she/it says

safn (b) ll. safnau - jaw

dim ond hyn a hyn - only so smuch

trugaredd (g) - mercy, compassion

gloddesta - to revel, to carouse, to feast; (g) revelry, carousal, feasting

rhyddid (g) - freedom, liberty

fforddiadwy - affordable

cyfrodeddu (cyfrodedd-) - to intertwine, to twist together

cyffwrdd (cyffyrdd-) â - to touch

yn y bôn - basically, essentialy

chwiban (g, b) ll. chwibanau - whistle (sound)

llongyfarch (llongyfarch-) - to congratulate; to greet joyfully

Dere i mewn! - Come in! (De Cymru)

gweithdy (g) ll. gweithdai - workshop

trallodi (trallod-) - to afflict, to trouble

treuliau - expenses

seneddol - parliamentary

ysgythriad (g) ll. ysgythriadau - engraving, etching

sgrechlyd - screeching, shrieking

neuadd (b) ll. neuaddau - hall

llawes (b) ll. llewys - sleeve

tasgu (tasg-) - to burst out, to splash, to spurt

gwres (g) - temperature, heat, heating, warmth

sicrwydd (g) - certainty

llesg - weak, frail, faint, dispirited, faint-hearted

etifedd (g) ll. etifeddion - heir

llaeth enwyn (g) - buttermilk

lloffa (lloffa-) - to glean

eitha[f] (fel adferf o flaen ansoddair) - fairly, quite, rather

dylanwad (g) ll. dylanwadau - influence

mefus - strawberries

cadw at addewid - to keep a promise

Does dim dwywaith amdani! / Does na'm dwywaith amdani! - There are no two ways about it! / There's no doubt about it!

cromen (b) ll. cromenni - dome

ceulo (ceul-) - to clot, to coagulate, to congeal

cydlynol - coherent, cohesive

cymwys - suitable, appropriate, qualified, eligible

anaeddfedrwydd (g) - immaturity, unripeness

dychwelyd (dychwel-) - to return, to turn back

rheswm (g) ll. rhesymau - reason, motive, grounds

dod i law - to come to hand, to come into one's possession

ysbïwr (g) ll. ysbiwyr - spy

pistyll (g) ll. pistylloedd - waterfall, stream of falling water, water jet, water column

mynachlog (b) ll. mynachlogydd - monastery

hydawdd - soluble (of substance in solvent)

swyddogaeth (b) ll. swyddogaethau - function, role, office

hapchwarae (hapchwarae-) - to gamble

amgryptio (amgrypti-) - to encrypt

tagfeydd u. tagfa (b) - bottlenecks, (traffic) jams

marblen (b) ll. marblis - marble (spherical toy)

'Sdim dal / Does dim dal - There's no telling / knowing

cas - nasty, disliked, hated

praidd (g) ll. preiddiau - flock

maen (g) ll. meini - stone, rock

symbylu (symbyl-) - to goad, to spur, to motivate, to urge

cynhyrfu (cynhyrf-) - to become excited, stirred, agitated, uneasy

dirnad (g) - understanding, comprehension

us (g) - chaff, husks