To stress the key part of an argument, move it to the front of the sentence and follow it with 'sy'/'sy'n' when the fronted word is the subject, or 'yw'/'ydy' when you are identifying it. Everything else follows, and the ordinary 'mae' disappears.
Examples
- Yr economi sy'n peri'r pryder mwyaf.
- It's the economy that causes the greatest concern.
- Diffyg tystiolaeth yw gwendid mawr y ddadl hon.
- A lack of evidence is this argument's great weakness.
- Am y rheswm hwnnw dw i'n anghytuno'n llwyr.
- For that reason I completely disagree.
The mistake to avoid
Leaving 'mae' in place after fronting — 'Yr economi mae'n peri...' is wrong; use 'sy'n'.
Where this fits in
Dadlau rhesymegol, Welsh C1: Build a logical argument: claims, evidence, counterargument, persuasion.