That's the girl who made the poster!

'S e sin an nighean a rinn am postair

a | nach 

We use a in the middle of a sentence, and before a verb, to mean who, which, that . It's also known as the relative particle. When we use it join to two independent clauses with a positive verb, we use the independent form of the verb. 

To join a negative clause in this way we use nach (who, which, that not). As with the question càit(e) , nach uses the dependent form of verbs. 

'S e sin an nighean.  + Rinn i am postair.  = 'S e sin an nighean a rinn am postair. 
That's the girl.  + She made the poster.  = That's the girl who made the poster. 
Seo an duine. + Cha d' fhuair e duais. = Seo an duine nach d' fhuair duais. 
This is the man. + He didn't get a prize. = This is the man who didn't get a prize 

Play with the sentences until you are happy with a and nach . 

'S e seo an nighean

This is the girl who

a

a

'S e seo an duine

This is the man who