1. 'Tha' is for describing things. So colors, textures, sizes etc but also states of being ie what you are doing.
Tha an cat beag. The cat is small.
Tha an cù donn. The dog is brown
Tha mi ag obair. I am working.
2. 'Is' equals. Is is for equating two nouns. So if you can write your sentence as an equation then you should use 'Is'.
Is mise an tidsear. mise= teacher. (I am the teacher)
Is Iain an saor. Iain = an saor. (Iain is the carpenter)
3. If your sentence starts with 'It is a ____' Use 'Is', but it is going to look like this: 'S e (Is e). And you will most likely follow this pattern: " 'S e _______ a th' ann. " put your noun in the blank space.
'S e bò a th' ann. It is a cow.
'S e truca a th' ann. It is a truck.
4. Talking about the weather. The tip here is that you can talk about the weather with both 'tha' and 'is' sentences and the trick is to have your whole sentence follow the 'tha' format or your whole sentence follows the 'is' format.
Tha i brèagha. It is lovely.
Tha an latha brèagha. The day is lovely.
'S e latha brèagha a th' ann. It is a lovely day.
5. 'S ann is also a form of 'is' and is often use for emphasis. If you want to emphasis things like location and time you can use 'S ann to emphasize this part of the sentence. This transforms a 'tha' sentence into a 'is' sentence.
Tha i à Alba. She is from Scotland
'S ann a Alba a tha i. It is from Scotland that she is.
Chunnaic e Sìne an-dè. He saw Sìne yesterday.
'S ann an-dè a chunnaic e Sìne. It is yesterday that he saw Sìne.