"The food is tasty!"
I wrote האוכל הוא טעים and it is marked as wrong. When can I use הוא to indicate "is", and when can I not?
האוכל הוא טעים is a less acceptable manner of speaking in Hebrew. It's not entirely wrong, but generally I guess it'd better not be taught to construct sentences like that. 'is' would usually not have a direct or literal meaning in Hebrew, as the use of the verb 'be' is very different.
ריגא אדרים But don't confuse the world. Someone might actually learn that :D