"Mi wnaeth hi weld."

Translation: She saw.

[Michael8427]

Please, what is the significance of the pronoun 'mi' in this sentence

[ibisc]

In this usage mi (or fe) is not a pronoun but an affirmative particle - it just indicates that the verb which follows it is an affirmative statement, not a question or a negative. The use of mi/fe is entirely optional, although mi is common in the north-west Wales dialects. If mi/fe is used, then the verb mutates. Sometimes the verb mutates even if the particle is not actually being used.

• mi welais i... I saw...
• fe welais i ... I saw...
• gwelais i.... I saw...
• welais i... I saw...

[cwtch_y_ddraig]

To me, this says 'she did see'. 'She saw' would be 'mi welodd hi'.

[ibisc]

They are both valid ways of expressing the simple past tense.