Mae'r gair Mo yn tarddu o ‘ddim’ + ‘o’ – (ddi)m o. Byddwn ni'n defnyddio Mo i ffurfio negyddol berfau cryno, hyny yw, fel rheol, yn yr amser gorffennol, neu yn yr amser dyfodol, e.e.

gweld + fi – Gwelais i / Gwela i
I saw / I will see

Dim ond gyda'r canlynol y byddwn ni'n defnyddio 'Mo'

1) Gwrthrychau pendant, hynny yw, ‘the + something' – y bwyd, y cwestiwn, yr e-bost, e.e:
Bwytais i fara
I ate (some) bread

Fwytais i ddim bara
I didn’t eat (any) bread

Bwytais i’r bara
I ate the bread

Fwytais i (ddi)mo’r bara
I didn’t eat the bread (Yn llythrennol – I didn’t eat nothing of the bread)

2) Rhagenwau personol ac enwau priod – fi, ti, fe, hi, ni, chi, nhw, Ffred, Siân, e.e.

Gwelais i ti
I saw you

Welais i (ddi)mohonot ti
I didn’t see you

Gwelais i Siân
I saw Siân

Welais i (ddi)mo Siân
I didn’t see Siân

* Bydd rhaid ychwanegu terfyniadau at fôn yr arddodiad 'o' (sef, 'ohon-') pan fyddwn ni'n defnyddio rhagenwau personol, e.e.

Welais i mohonot ti /o fe/i hi/och chi/yn nhw
I didn’t see you/him/her/you/them

Byddwn ni'n hegor ail ran y ffurfau fel 'mo/honot' ar lafar, e.e.

Welais i mohonot ti > Welais i mo ti/Welais i ddim o ti

3) Rhagenwau meddiannol – fy, dy, ei, ein, eich, eu, e.e.

Gwelais i dy frawd di
I saw your brother

Welais i (ddi)mo dy frawd di
I didn’t see your brother

Mo comes from ‘ddim’ + ‘o’ - (ddi)m o. It is used when forming the negative of short form verbs, i.e. usually the past or future tenses, e.g:
gweld + fi – Gwelais i / Gwela i
I saw / I will see

It is only used with the following:

1) Definite objects, i.e.‘the’ something - y bwyd, y cwestiwn, yr e-bost, e.g:
Bwytais i fara
I ate (some) bread

Fwytais i ddim bara
I didn’t eat (any) bread

Bwytais i’r bara
I ate the bread

Fwytais i (ddi)mo’r bara
I didn’t eat the bread ( Lit. - I didn’t eat nothing of the bread)

2) Personal pronouns and names - fi, ti, fe, hi, ni, chi, nhw, Ffred, Siân, e.g.

Gwelais i ti
I saw you

Welais i (ddi)mohonot ti
I didn’t see you

Gwelais i Siân
I saw Siân

Welais i (ddi)mo Siân
I didn’t see Siân

*An ending has to be added to the root of ‘o’ - ‘ohon’ when the personal pronouns are used, e.g:

Welais i mohonot ti /o fe/i hi/och chi/yn nhw
I didn’t see you/him/her/you/them

The above are often omitted in informal speech, e.g:
Welais i mohonot ti > Welais i mo ti/Welais i ddim o ti

3) Possessive pronouns – fy, dy, ei, ein, eich, eu, e.g.

Gwelais i dy frawd di
I saw your brother

Welais i (ddi)mo dy frawd di
I didn’t see your brother