"Is it important? It is."
Translation:An bhfuil sé tábhachtach? Tá.
Perhaps if it were like this:
Rud tábhachtach is ea é, an ea?
To which you would reply Is ea.
Note that Is ea is written but 'Sea is spoken.
Where you use synthetic forms normally, you can use a synthetic form in your answer, so táim and táimid, but where you use the analytic form, the pronoun isn't required and isn't usually present in the response - just tá or níl.
If the pronoun is included, it is used for emphasis - An bhfuil sé réidh? Tá sé - "Is it ready? Oh, it's ready"
And for clarity, tá and níl are only answers to an bhfuil?. To answer "yes" to "Do you eat cheese?" - An itheann tú cáis?, you say ithim, not tá.
For asking a question (in the present) that has a Tá or Níl answer you must use An bhfuil.
An itheann tú cáis?, An maith leat Duolingo?, An as Corcaigh thú?, An mbreathnaíonn tú ar "Vikings"? An dtuigeann tú?
There are lots of present tense questions that do not use an bhfuil.
Questions that start with "Do you" in English usually don't start with an bhfuil in Irish ("do you have" being a major exception, because "have" requires the verb bí in Irish).
What you say is true and thanks for pointing it out but in the context of the question asked by Becky An bhfuil is the form required as opposed to just making a statement and putting a question mark at the end. I have modified my answer to take account of your point.
Becky's question (about tacking a question mark onto a simple statement and calling it a day) would apply to "Itheann tú cáis?" just as much as to "Tá sé tábhachtach?".
You have edited your answer above to clarify that you were only talking about questions with a Tá or Níl answer but the general point is that you need to use an interrogative an at the start of a present tense question.
(it's an old question, but worth updating, because I know from other comments that Becky made at the time that she didn't fully appreciate that an bhfuil is only the interrogative form of tá, and I know that she's not the only beginner who has found that confusing).
If you mean that to ask the question "Is it important?" do you use Tá sé tábhachtach the answer is No. Tá sé tábhachtach is just a statement that says "It is important", not "Is it important". To ask a question you use the question word An followed by the dependent form of the verb. In this case the dependent form is fuil. The question word An eclipses the verb so it becomes An bhfuil.
Tá sé tábhachtach = It is important
An bhfuil sé tábhachtach? = Is it important?
If you mean that to answer the question "Is it important? do you use Tá sé tábhachtach the answer is again No. Irish does not have a direct word for "Yes" or "No". Instead to answer "Yes" you just use the independent form of the verb that was used in asking the question. In this case the independent form is Tá.
Question: An bhfuil sé tábhachtach?
Reply (in the affirmative): Tá.
Note that the verb bheith = "to be" is irregular which complicates matters further.