"These are two cats."
Translation:Seo dà chat.
That explains the i. And the reason that there is no h is that the basic word is cat. The h is added because some words, including aon 'one' and dà 'two' cause lenition.
If you are confused by any resemblance to the French word for a cat, this is a coincidence. C changed to ch (pronounced sh) in French perhaps 1000 years ago, and so there are lots of words that have a c in Gaelic and a ch in French:
Note that the English words that have a ch come from French but the others don't.
I'm afraid the only answer is that different languages work in different ways. This is a course in colloquial Gaelic, not theoretical linguistics and you need to accept new structures as you meet them. These pages are full of people saying 'I thought it was x and now you're saying it's something else'.
In this particular case, the Western concept of grammar was invented by the Romans some 2000 years ago in order to describe how Greek works and one of its tenets, such as 'all sentences must include a verb' do not apply to all languages. There are two verbs 'to be' in Gaelic, as in Spanish, and the other one, is really doesn't obey the rules we are used to about what a verb is and when it is necessary. Some grammarians make up rules about the verb being absorbed or seo 'being' the verb, but they are really just trying to pretend Gaelic is Greek. So just learn and enjoy.