Gaelic has two separate to be verbs – the copula is and the substantive verb bi. The substantive verb bi (and its forms like tha, (bh)eil, bha, robh, bidh, etc.) are used only to describe something or state its whereabouts (its predicate is always an adjective, an adverb, or a prepositional phrase):
- tha e cho saor – it is so cheap (adjective),
- tha an cat air a’ bhòrd – the cat is on the table (prepositional phrase),
The copula is used to state what or who something/someone is (its predicate can be a noun phrase):
- is mise Ruairidh I am Rory (Rory is a noun),
- is cat e it is a cat (a noun).
(but then instead of simple copula Gaelic often uses more complex idiomatic phrases to state what or who something is: is e cat a tha ann it is a cat, lit. it is a cat that is in it, Pòlannach a th’ annam I am a Pole, a Pole is what is in me)
The is part in the meaning this is of seo… is only the copula, it is short for is e seo… and it needs a noun phrase as its predicate – it is only used to state what or who ‘this’ is:
- seo cat or is e seo cat: this is a cat
- seo m’ athair or is e seo m’ athair: this is my father
To describe ‘this’ you still use the substantive verb:
- tha seo mòr this is big,
- tha seo cho saor this is so cheap.
See also my answer under the sentence "Tha sin dona".
(Actually you could use copula to describe something, but this is rather archaic or poetic usage, and the syntax is different, the adjective or an adverb must go directly after the copula, so maybe is saor e seo or is saor an rud seo for this (thing) is cheap – there is one common phrase retaining this syntax: is math sin for that is good)