"Water is necessary."
Translation:Tá uisce riachtanach.
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
bí can be, in certain sentences, like tá sé ina dhoctúir. But that has a different meaning. Do you have any other examples?
Riachtanach is an adjective. You're describing what water is, not classifying or identifying it as something, which generally requires another noun.
In Spanish one uses ser, not estar, for "necesario". Hence I used "is" instead of "bí". It is clearly a bit different in Irish.
Yes. The comparison with Spanish isn't one-to-one. generally, if you're linking two nouns (including pronouns), you use is. Otherwise, bí.