Ceacht 17:
ní raibh	was not, were not	[neg.marker ]+[ dep. form (past tense)  irreg. v.]	
ní raibh sé	he wasn't	["] + [pron.]	
ní raibh siad	they weren't	["] + [pron.]	
na	the (plural)	pl. def. art.	
na buachaillí	the boys	[pl. def. art.] [n. ]	
na cailiní	the girls	[pl. def. art.] [n. ]	
na leanaí	the children	[pl. def. art.] [n. ]	
na gardaí	the guards	[pl. def. art.] [n. ]	
na múinteoirí	the teachers	[pl. def. art.] [n. ]	
na rudaí	the things	[pl. def. art.] [n. ]	
na rudaí sin	those things	[pl. def. art.] [n. ]	
leis	with	prep. pron.	
ar scoil	at school	[prep. phrase used adverbally]	
inniu	today	adv.	
dána	bold/bad (misbehaving )	adj.	
ag caint le	speaking to	“ag” + [vn.] + [prep. pron.]	
sa scoil	in the school	[prep.pron(=i+an)] + [n f2]	
inné	yesterday	adv.	
buachaill dána	a bold boy/ a bad boy	[n. m3] + [ adj.]	
a dhaidí	daddy (addressing him)	[voc. case # len.# ]	

raibh:  This is the dependant form (past tense) of the irregular verb bí which we've been dealing with mostly so far in this course.  The dependant form of a verb is required after the negative marker ní or the interrogative particle an.  It is also required following cá and nach.

plural nouns: na (the):  Lesson 10 gave us an introduction to the plural definite article (na) and the plural form of most of the nouns used in this lesson.

na rudaí sin (those things):  In Irish, when we wish to talk or write about "this X" or "that X" we use the formula:
[an] + [X] + [seo] = this X
[an] + [X] + [sin] = that X

So to identify one particular item we use the singular definite article (an), followed by the noun, followed by the adjective seo for this or sin for that. The Irish word for "thing" (singular) is rud, so we get the construction:
[an] + [rud] + [seo] = this thing
[an] + [rud] + [sin] = that thing

However, if we wish to identify more that one item we use the plural definite article (na), followed by the plural noun, followed by the adjective seo or sin as above, giving us the formula:
[na] + [Xs] + [seo] = these Xs  /  [na] + [Xs] + [sin] = those Xs

The plural of rud is rudaí so we get the construction:
[na] + [rudaí] + [seo] = these things  /  [na] + [rudaí] + [sin] = those things