Source: http://www.phouka.com/gaelic/book03/exe111-1.htm
Timestamp: 2019-04-25 18:19:34+00:00

Document:
§ 627. We have seen that in English sentences where the verb to be is followed by a pronoun, a proper noun, or a noun with the definite article or the possessives my, thy, etc the verb to be must ALWAYS be translated by is.
§ 628. We have also met sentences where the verb to be was followed by (a) an adjective, as the day is cold; (b) a noun with the indefinite article a or an; as , he is a man, it is a hot day.
§ 629. Now, in sentences of this kind the verb to be is represented in Irish sometimes by atá and sometimes by is. Up to this we have used only atá, as , atá an lá fuar; atá sé 'na ḟear, atá sé 'na lá te. The idiomatic use of the preposition in, in the last tw sentences, is familiar to our students.
§ 630. But we can also use is and say, is fada an lá, is fear é, is lá te é etc.
A. The collocation of the words. Up to this the order of the words was (I) verb (2) nom. case (3) adjective or noun, which in English sentences followed the verb. But now we see in sentences like is fada an lá, is fear mé, the order of words is (I) verb (2) adjective or noun which in English followed the verb (3) nominative case.
§ 632. B. There is also a difference in MEANING between is and atá, which we shall try to illustrate by examples. The reason of difference is that atá means is now and is means is always (or is without any reference to time or circumstances.) Take the word bacaċ (bok'-ăCH) Munster (bok'oCH'), lame. Atá mé bacaċ means "I am lame", ie, at present and for a time only. Is bacaċ mé means "I am lame permanentlγfor life. I am a cripple." Hence the word in such a sentene is equivalent to the noun "cripple". Often used for beggar.
§ 633. So Atá e fuar, "it is (now) cold" often the same as "it has become cold", as atá an lá fuar, the day is now cold. But, is fuar é "it is (always) cold" would not be said oa nything that is sometimes cold and sometimes not, but of something that is always cold (or, at all events, the notion of a present state of coldness is not in the mind). Hence, is is the verb general ysed in proverbs: as, má's fuar an teaċ-taire (taCH-thărĕ) is fuar an freagra (fra'gră). If (má's = má is) the messenger is cold (carelesss) the answer is cold.
Another way of knowing when to use is and when to use atá. We may take it that is is the word most generally used where 'is' is used in English. When we wish to say that two things are identical, as "John is the king" or "this is a fine day", "this day is (a) fine (day)" we use is, is é eaġán an rí", "is lá breaġ é so." "is breaġ an lá é so". But when "is" means exists or expresses a state or condition, atá is used. When the statement would be made in answer tot he question "what is—?" "who is—?", "Of what kind is—?" we use is. When the questio nis "How is—?" "Where is—?" "In what condition, etc is—?" we use atá.
§ 635. Whenever in English the verb to be is followed by a preposition, atá is the Irish verb to be used. This follows from the mature of prepositions; for, when we say that some one or some thing is at a place, on a place, from a place, is always means is now, or has reference to a state or condition.

References: § 628

§ 629

§ 630

§ 632

§ 633

§ 635