There is no my bedroom in this sentence. That would be most likely expressed as an seòmar-cadail agam, something like the bedroom of mine, very literally the room-of-sleeping by me.
But you don’t have that in the sentence, here it breaks down as:
- tha – is, the present tense of verb bi to be,
- seòmar-cadail blàth – a warm bedroom, no the before it – it is the subject of the sentence, the thing that is,
- agam – at me – this is the predicate of the verb is, the thing that in English goes after is, eg. in he is there you have he as the subject and there as the predicate.
So the sentence fairly literally means a warm bedroom is at me and when something is at you in Gaelic, it means that you have it. Thus the meaning of this sentence is I have a warm bedroom.
If you wanted to say my bedroom is warm you would need my bedroom to be the subject and warm to be the predicate:
- you would start the same as the previous sentence: is – tha – because the verb goes first in Gaelic,
- my bedroom – an seòmar-cadail agam,
- warm – blàth.
Giving you: tha an seòmar-cadail agam blàth.
It’s similar as the previous sentence, it uses the same words (though it adds an the in there), but the structure is different.
And if you had a feminine noun then the adjective would change. Eg. I have a pretty park would be tha pàirc bhrèagha agam while my park is pretty would be tha a’ phàirc agam brèagha – because in the first sentence the adjective brèagha would attribute a feminine noun directly (it’s … pretty park) and thus would lenite, in the second sentence the adjective is a predicate to the verb (it is …park is pretty and not just pretty park) so it would not be lenited.