is is use for classification/identification sentences (at this level), whereas bí (tá is a form of bí) can't be used for those. It's very similar to ser versus estar in Spanish
The reason I asked is because Ta seems to be used for classification/identification (_ is __-type sentences) a lot, when I thought Is would be used. I don't know Spanish, so I don't know the difference between ser and estar.
I know "I am a doctor" is "is doctuir me" (still can't do accents in comments), but why is the sentence above (Water is important) not "is uisce riachtanach?"