Hi @Susanne
Bod is used for ‘that’ when the second half of the sentence, on its own, would use the present tense.
Because in this situation, the ‘bod’ reflects both the tense and the ‘that’ combined (and might have the ‘person’ associated with the second half of the sentence, hanging off of it eg…that I…that you…etc) - it needs to be there.
For other tenses in the second half eg would, should, could, will, did - y or yr is technically used for ‘that’ - but this one, like English, is more often than not dropped in speech.
In this situation the verb that follows handles the tense and the ‘person’ that follows, as normal.
Eg
I think (that) I should go now
Dw i’n meddwl (y) dylwn i fynd nawr
I think (that) I would like to go now
Dw i’n meddwl (yr) hoffwn i fynd nawr
I think that you are speaking it very well
Dw i’n meddwl bo’ ti’n siarad hi’n dda iawn.
Hopefully that makes sense! 
Rich 