Ceacht 3:
tá sé	he is 	[v] + [pron]	
mór	big	a	
beag	small 	a	
sean	old	a	
óg	young	a	
tinn	ill	a	
sin	that	dem pron	
leat	with you	prep pron	
is	is	copula ( linking verb )	
ar chor ar bith	at all	adv	
is maith sin	that's good	[pres tense of copula ]+[a] + [dem pron]	

So far we have seen that the Irish verb "tá" can be translated as the English verb "is", and that its negative form is "níl"

An important feature of lesson 3 is the introduction of an alternative word for "is", namely "is".
This is known as the COPULA.  Its use demonstrates one way in which the basic sentence structure, which we saw in the first two lessons can vary, and we see an example on page 11:

[is] + [maith] + [sin] - that is good.

Used in this way we are bringing the adjective forward, sometimes referred to as "fronting", in order to add emphasis to it.  

Notice also that the adjective "maith" is used WITHOUT the particle "go" which was mentioned in lesson 2.