Mae 'Pe' ac 'Os' fel ei gilydd yn golygu 'If''. Dim ond 'Pe' y byddwn ni'n ei ddefnyddio yn yr Amodol, a dim ond 'Os' fel arall. Yn llawn, rydyn ni'n ysgrifennu, ac yn dweud, er enghraifft, ‘Pe taswn i’ ('It I were to be' yn Saesneg). Ond, byddwn ni'n hepgor y seiniau 'pe' a 'ta' fel arfer ar lafar, hynny yw:

Pe taswn i > ’Swn i
(ysgrifinnedig) > (ar lafar)

Mewn geiriau eraill, y gair talfyredig ' ’swn ' yn cynnwys y syniad o 'pe'.

Fel arfer, bydd rhaid defnyddio 'yn traethiadol' ar ôl 'Bod' yn yr Amodol i gysylltu dwy ran y frawddeg â'i gilydd, e.e.

(Pe) (ta)sai hi + yn + mynd > ’Sai hi’n mynd 'If she went' (Yn llythrennol, 'If she were to go')

Unwaith eto, yn yr iaith Gymraeg, arwydd yr Amodol yw'r sain 'e':

Ysgrifennedig     Ar Lafar
’Swn i                   ’Sen i
’Sai fe                   ’Se fe
’Sai fe                   ’Se hi

Beth yw ystyr y ffurfiau hyn yn Saesneg? Wel, mae sawl ffordd o ddweud yr un peth, e.e.

’Swn i’n mynd...
If I was to go
If I would go
If I went / If I did go
If I were to go
Were I to go
Should I go

Pe’ is the conditional ‘if’ (used instead of ‘os’ which is used with the present and future tenses). It is not usually pronounced in speech. Neither is the ‘ta’ from ‘taswn, etc.

So ‘pe taswn i’ becomes ’swn i in speech. In effect, the ’swn i includes the ‘if’.

As with other ‘bod’ verbs an yn or ’n is usually required to stick them to the rest of the sentence

e.g: ’sai hi’n mynd.
e’ is the key sound in this tense -    ’swn i    pronounced     ’sen i
’sai fe / hi           pronounced     ’se fe / hi

You could think of these as ‘if’ being used with a past sense,  e.g. with ‘mynd’ - to go:
If I was to go                     ’Swn i’n mynd
If I were to go                    ’Swn i’n mynd
If I would go                       ’Swn i’n mynd
If I went / did go                ’Swn i’n mynd

Yn llawn / In fullYnganu / Pronounced
Saesneg / English
(pe) (ta)swn i’sen eeIf I would / were / did
(pe) (ta)set ti’set teeIf you would / were / did
(pe) (ta)sai fe’seh vehIf he would / were / did
(pe) (ta)sai hi’seh heeIf she would / were / did
(pe) (ta)sen ni’sen neeIf we would / were / did
(pe) (ta)sech chi’sech cheeIf you would / were / did
(pe) (ta)sen nhw’sen nhooIf they would / were / did