Here the verb dangos has a personal suffix (ending) but we do not know who the person responsible for the action is.  We should therefore use an impersonal suffix.

 -wyd,-ir and -id are the most commonly used suffixes for impersonal verb forms.

The impersonal verb forms can be shown as follows:

-ir	Gwelir	Present Tense	Gwelir blodau yn y gwanwyn
-wyd	Gwelwyd	Past Tense	Gwelwyd eira yn y gaeaf
-id	Gwelid	Imperfect Tense	Gwelid gwiwerod coch mewn coedwigoedd

When using concise forms of the verb, the normal sentence structure in Welsh is Verb + Subject + Object, e.g.

Rhedodd y ferch y ras.

Ysgrifennodd y bardd gerdd.

Dylai'r plant gael gwersi.

However, sentences which include impersonal verbs do not follow this pattern. 
When using the impersonal tense, the object follows the verb and the subject/person completing the action is included after gan, e.g.

Rhedwyd ras gan y ferch.

Ysgrifennwyd y gerdd gan y bardd.

Note also that there is no soft mutation after an impersonal verb (unlike personal verbs), e.g.

Ysgrifennodd y bardd gerdd.

Ysgrifennwyd cerdd gan y bardd.