how to use the word "demain'
In English words like 'yesterday' and 'tomorrow' can function as both nouns and adverbs and so we can have sentences such as:
Tomorrow is Sunday (here tomorrow acts as a noun)
I will do that tomorrow. (here tomorrow acts as an adverb)
Am I right in thinking that in French the word demain only acts as an adverb and therefore it is incorrect to use the structure
Demain est ...
instead you need to say things like
il fera beau demain / demain il fera beau - tomorrow will be sunny
Demain, c'est la dernière étape - tomorrow is the last stage
Demain nous sommes dimanche. - tomorrow is Sunday
Demain c'est dimanche. - tomorrow is Sunday