"This is quite short."
Translation:Tha seo gu math goirid.
Seo Anna gives this is Anna, seo cù gives this is a dog, etc., but it only works with noun phrases. You use it to identify or classify this (those sentences are actually a shorthand for something like is e seo Anna, is e seo cù with the copula verb is – but this verb in many situations, like in this case, can be omitted).
But to describe things you typically use the bi verb (tha is its present tense).
So if you want to say this is there, this is big, this is quite short, you need to use tha.