"This is a good picture!"
Translation:Seo dealbh math!
It’s not a difference between seo and sin but between two types of to be that Gaelic differentiates, look at my answer in the Tha sin dona thread.
When the predicate (the thing tha comes after is in English) is a noun phrase (you define or identify something), you use seo X and sin X:
- seo m’ athair this is my father (my father is a noun phrase),
- sin bòrd that is a table (a table is also a noun phrase),
- sin dealbh math that is a good picture (a good picture is a noun),
- seo dealbh dona this is a bad picture (a bad picture is a noun phrase).
When you describe someone or something or their whereabouts, you use the bi verb (and its forms like tha, chan eil, bidh, etc.):
- tha seo dona this is bad (bad is an adjective),
- tha sin air a’ bhòrd that is on the table (on the table is a prepositional phrase).
You’ll find the exact same difference eg. with tha mi(se) toilichte I am happy vs. is mise am fear sin I am that man, or tha cat mòr a cat is big vs. is e cat mòr a tha ann it is a big cat.
This sentence is this is a good picture and not this picture is good – they are two different sentences (with very different structure in Gaelic!).
If it were this picture is good then the answer would be – almost as you wrote – tha an dealbh seo math (but notice that an dealbh seo this picture has an there).