You would use "Tha seo ..." when describing something, followed by an adjective (e.g. Tha seo buidhe = This is yellow; Tha seo ro fhuar = This is too cold). "'S e seo" (note the apostrophe goes before the S, since it's an abbreviation of "is") is used when identifying something or someone, followed by a noun. It is very often shortened to "Seo" (e.g. ('S e) seo an t-àite ceart = This is the right place; ('S e) seo m'athair = This is my father).