Bod
Doeddwn i ddim yn gwybod bod...
Rydw i'n gwybod bod...
In these examples, bod is used to link two clauses together in the sentence and in particular to link a noun clause as the subject of a verb in the main clause, e.g.
Rydw i'n meddwl bod y llyfr yn dda.
I think that the book is good.
In English, the bod appears to mean 'that'. However, it is not a direct translation of 'that'. It operates as a link between the sentence's main clause and its sub-clause and presents further information relating to the sentence's object.
In the present and imperfect tenses, the bod in these types of sentences cannot be omitted in Welsh, e.g.
Rydw i'n meddwl bod y llyfr yn dda.
I think that the book is good.
Rydw i'n meddwl ei fod e'n dda.
I think that it is good.
If two separate sentences are joined together, as in the examples below, mae has to be changed into bod, e.g.
Rydw i'n meddwl. Mae'r bachgen yn ddoniol
Rydw i'n meddwl bod y bachgen yn ddoniol.
I think that the boy is funny.
Rydw i'n meddwl. Mae hi'n ddiflas.
Rydw i'n meddwl ei bod hi'n ddiflas.
I think that she/it is miserable.