In this section we will summarise the gramatical rules for use of treigladau which depend on the sentence structure.
The most important rule is that a treiglad meddal is applied where possible after the subject of the sentence. For example, using a short form verb:
Prynodd Alys docynnau i'r gyngerdd. tocyn ticket
Alys bought tickets to the concert. subject: Alys
Roiff yr Athro ddarlith am ei waith ymchwil. darlith lecture
The Professor will give a lecture about his research. subject: yr Athro
The same rule applies when a long form verb is used. This may include a tense of the verb 'gwneud':
Gwnaeth yr adeiladwr beintio'r ffenestr. peintio paint
The builder painted the window. subject: the builder
Gwnaiff yr adeiladwr beintio'r ffenestr.
The builder will paint the window.
An alternative long form verb uses a tense of 'bod':
Bu'r adeiladwr yn peintio'r ffenestr.
The builder painted the window.
Bydd yr adeiladwr yn peintio'r ffenestr.
The builder will paint the window.
The connecting term 'yn' is added with a bod tense. This cannot take the treiglad meddal which is normally applied after the subject.
A treiglad is not applied to a verb noun following 'yn':
Mae Siôn yn beicio i'r coleg. Siôn cycles to college.
However, a treiglad meddal is applied when 'yn' is followed by a noun or adjective:
Mae Siôn yn fyfyriwr. Siôn is a student. myfyriwr student
Mae Siôn yn flinedig. Siôn is tired. blinedig tired
In an exception to the rule, 'yn' does not cause mutation of nouns or adjectives beginning with 'll' or 'rh'
Maen nhw'n llyfrau. They are books.
Mae e'n rheolwr. He is a manager.
Maen nhw'n llyfn. They are smooth.
Mae'n rhad. It is cheap.
Note that 'yn' is not added for a tense of 'bod' if the subject is followed by a preposition, for example:
Mae Siôn gyda'i ffrindiau. Siôn is with his friends.
In some sentences a treiglad meddal is applied after the assumed subject which carries out the action, even though this may not be the true grammatical subject. For example:
Mae'n rhaid i Siôn fynd. Siôn must go. (Siôn is the assumed subject)
Bydd rhaid iddo fe fynd. He will have to go. ('He' is the assumed subject)