"I am not tired, friends. You are tired!"
Translation:Chan eil mise sgìth, a chàirdean. Tha sibhse sgìth!
The practical answer is that you just have to learn these expressions, like you have to learn lots of expressions in any language.
The technical, but not very helpful, answer is that you are an adjective (tha mi sgìth) but a noun is on you (tha an t-acras orm 'the hunger is on me').
In practice this is no help at all as you cannot tell which are the nouns and which the adjectives - unless you know there is an article present. In fact it works the other way round. These expressions help to determine which are the adjectives and which the nouns.
I finaly understand "I have to learn the expressions" thing! I have been learning close to a year and I am getting used to the expressions! I have written out the sentences and I dont have to refferance the old lessons . They are eched on my mind. Great progress! Thanks
The basic answer is that these words are emphatic and you might typically use them when contrasting two people as here, but there is no hard and fast rule. But, in addition, there is no equivalent in English, so there is no clear way in the English to indicate they are wanted in the Gaelic. For that reason I think they have to accept mi and thu/sibh even though they deliberately created the contrast in order to encourage you to use these emphatics.
You just have to second guess them and recognise sentence with clues they want you to use the emphatics.