Adverbs describe and modify a verb, an adjective or another adverb. If a word answers questions such as how, how many, how much, how often, what, when, where, which, who, why, etc, then it is an adverb.

There are two types of adverb:

●Adverbs formed from adjectives
The link word yn (contracted to ‘n after a vowel) turns an adjective into an adverb and also causes a soft mutation*. There are countless possible examples of this process, for instance:

You speak Welsh = Rydych chi’n siarad Cymraeg
You speak Welsh well = Rydych chi’n siarad Cymraeg yn dda (da = well)

It is raining = Mae’n bwrw glaw
It is raining heavily = Mae’n bwrw glaw‘n drwm (trwm = heavy)

She is walking = Mae hi’n cerdded
She is walking slowly = Mae hi’n cerdded yn araf (araf = slow)

He understands = Mae e’n deall
He understands perfectly = Mae e’n deall yn berffaith (perffaith = perfect)

●Adverbs that exist as standalone words
They do not require yn and all indicate time, place, frequency or degree

KEY
ˢ – causes a soft mutation
† – mutation has become fixed

Adref – Homewards
Allan – Out
Ar draws – Across
Ar hyd – Along
Bron – Almost/Nearly
Byth – Ever/Always
Ddoe† – Yesterday
Dwywaith – Twice
Dymaˢ – Here is/are; This is/These are
Dynaˢ – There is/are; That is/Those are
Echdoe – The day before yesterday
Efallai – Perhaps
Eleni – This year
Erioed – Ever/At all
Eto – Again
Fel – So/As
Felly – So/Thus
Heddiw – Today
Hefyd – Also/Too
Heibio – Beyond
Heno – Tonight
I fyny – Up/Upwards
I ffwrdd – Away
Isod – Below/Beneath
(y) Llynedd – Last year
Nawr – Now
Neithiwr – Last night
Nes – Until
Newyddˢ – Just
Nid – Not
O’r blaen – Before
Rhyˢ -Too
Uchod – Above
Unwaith – Once
Wedyn – Afterwards
Yfory – Tomorrow
Yma – Here
Ymhell – Far
Ymhellach – Further
Ymlaen – Forward/Onward
Yna – Then
Yna – There
Yno – There
Yn ôl – Ago/Backwards

*NOTE
In specific circumstances nine consonants are subject to the following soft mutations:
p → b
t → d
c → g
b → f
d → dd
g → –
ll → l
m → f
rh → r