"Your brother is responsible."
Translation:Tá do dheartháir freagrach.
There are eleven irregular verbs in Irish; the verb to be (bí) is one of them. For all other verbs, you only need to know the basic root of the verb in order to construct the present/past/future etc - but for the irregular verbs you need to memorise the individual tenses!
You met the present tense of bí in Basics 1: tá mé/táim, tá tú, tá sé, tá sí, tá muid/táimid, tá sibh, tá siad.
(This is really to Lancet, but there is no more "reply" indent room.)
Yes, I am familiar with conjugation and even irregular conjugation as concepts, but for those utter newcomers to Irish, like myself, a list of the irregular verbs might be useful, especially in cases like 'bi', where the infinitive bears no obvious etymological relationship to its conjugated forms. Just a hint would be good (like the one you have just given me, but in a general-access location, not buried in a discussion thread).