"That is a church."
It's because sin can have lots of different meanings. The tha sin ... construction is used for adjectives. "Sin" in this example means "this" as a pronoun (i.e. it is standing in for a noun). You can say:
- Tha sin fuar. > That is cold.
...because "cold" is an adjective. You could also say:
- Tha mi fuar. > I am cold.
In this example, the subject of the sentence isn't sin ("that") anymore, it's mi ("I").
On the other hand, sin can be used as a shortened version of the copular phrase is e sin... (meaning, "that is..."). The copula is in Gaelic is how you say that something IS something:
- Is mise Joanne. > I am Joanne.
- Is tusa Laura. > You are Laura.
- Is e sin eaglais. > That is a church.
The copular phrase "is e" is often shortened to 's e. When 's e is used in conjunction with words such as "sin", "seo", and "siud" for example, it is completely omitted:
- Sin eaglais. > That is a church.
- Seo Laura. > This is Laura.
- Siud am baga > That there is the bag.
So if you say Tha sin eaglais, it sounds like you're saying "That is church", with "church" as an adjective. So it doesn't make sense :)