Why no 'tha"?
Because there is no verb. This isn't a complete sentence--it's just a phrase, a noun and an adjective. It's just like saying "a big dog", "a sunny day", "a lovely cat", etc. There's no verb "to be" , so there's no verb tha.
Please explain the reason for the word order ☺
In Gaelic, adjectives come after nouns, instead of before.
What is the difference between greannach and gruamach?
Gruamach is more sad, whereas greannach is more angry :)
Why not tha gobhar greannach?
Because that means “A goat (some unspecified goat somewhere) is grumpy.”
gobhar greannach isn’t a sentence; it’s just a noun described by an adjective: “a grumpy goat.”
The bh in gobhar does not make a v sound?
Can I say something like, "Tha mi gobhar greannach"?
That sentence means, "I am a grumpy goat." Is that what you wanted to say?