The rule requiring a treiglad after the subject is applied using common sense, rather than by following grammatical rules too precisely. For example:
Mae'n rhaid i Dafydd werthu ei gar. gwerthu
Dafydd has to sell his car.
'Dafydd' is not strictly the subject of the verb 'mae' , but it is clear from the meaning of the sentence that he is doing the selling. A treiglad meddal is therefore applied after 'Dafydd'.
Commands represent a special case. Consider the sentence:
Ffoniwch feddyg ar unwaith! meddyg
Phone a doctor immediately!
There is no subject present in the sentence, but it is understood that the subject is 'you'. Using the normal Welsh word order, the sentence becomes:
Ffoniwch (chi) feddyg ar unwaith!
and a treglad meddal is applied to the complement which follows 'chi'.
The treiglad rule applies in the normal way when a verb noun is used with 'gwneud' to create a cwmpasog tense. For example:
Wnaeth Sioned dorri'r deunydd i wneud y llenni. torri
Sioned cut the material to make the curtains.
A treiglad meddal is applied immediately after the subject 'Sioned'.
The verb 'bod' is unusual in adding the marker 'yn' in certain cases. This happens when a verb noun is used to create a cwmpasog tense. For example:
Roedden ni'n gwylio'r ras.
We were watching the race.
The subject of the sentence is 'ni'. A treiglad meddal is not applied to the verb noun which follows 'yn'.
The verb 'bod' also adds 'yn' when the subject is followed by a noun or adjective, as in the sentences:
Mae'r bws yn brysur heddiw. prysur
The bus is busy today.
Mae ein cymydog yn wyddonydd. gwyddonydd
Our neighbour is a scientist.
In contrast to the case of verb nouns, a teiglad meddal is applied to the noun or adjective following 'yn'.
In other cases where the subject of a 'bod' verb is followed by a word other than a verb noun, noun or adjective, then the marker 'yn' is not added:
Mae'r tŷ wrth ochr y llyn.
The house is beside the lake.
The normal treiglad rule would apply to the complement following the subject.