Difríocht idir 'go' agus 'gur'

According to my dictionary:
As conjunctions 'gur' is used for past tense regular verbs while 'go' is apparently for the present and future tense of regular verbs, and past, present and future of irregular verbs. So I suppose:
"Ceapaim gur thuig sé" - I think that he understood
"Ceapaim go dtuigeann sé" - I think that he understands
"Ceapaim go dtuigfidh sé" - I think that he will understand
because tuig is regular

"Dúirt sé go raibh sí go hálainn" - He said that she was beautiful.
"Dúirt sé go bhfuil sí go hálainn" - He said that she is beautiful.
"Dúirt sé go mbeidh sí go hálainn" - He said that she will be beautiful after he shaves her head in her sleep.
because bí is irregular

'gur' is apparently used sometimes if the verb is 'is'/'ba' depending on it being followed by a consonant.
For example
"is féidir liom" -> "Ceapaim gur féidir liom" - "I think that I can"
"is mac léinn é" -> "Déir sé gur mac léinn é - "He says that he is a student."
"ba chigire é" -> "Cheap sí gur chigire é" - "She thought it was/would be an inspector"

but
"is amadán é" -> "Ceapaim gurb amadán é" - "I think that he is an idiot"
"b'óinseach í" -> "Cheap sé gurbh óinseach í" - "He thought that she was/would be a foolish woman"

If this ends up being bogus then blame the dictionary people. Unless the bold grammar is correct but the example sentences are weird, in which case it's my fault and please don't kill me.