"Blood is red."
Translation:Tha fuil dearg.
Teachers often forget to explain this but it is not the same in all languages. These 'predicative' adjectives (to use the fancy word) when you say something 'is red' etc. never change according to number or gender in the Celtic or Germanic languages, but they do in Romance languages like French and Spanish.
This is why you may be confused if you have learnt any of those.