"The lion is big."
It doesn't require a הוא because it's a noun-adjective sentence. You can say האריה הוא גדול but you don't have to. They mean the same thing.
You can say האריה הוא חיה גדולה. Notice the interesting thing that happens here! אריה is male but חיה is female. גדולה is an adjective attached to חיה so agrees with it in gender. But the copula agrees with the אריה. That is because, in a noun = noun sentence, especially when using a pronoun as a copula (זה copulas work a little differently), the first noun is considered the subject, and the copula agrees with it. If I asked you, what is that big animal? You could say: החיה הגדולה היא אריה.
You can say זה הוא האריה הגדול. Usually, זה הוא contract to make זהו. You can write: זהו האריה הגדול.
i believe so. (based on the tips which you can accessed through Duolingo desktop browser version). to quote directly from the tips: When adding definite articles to nouns with adjectives, both the noun and the adjective receive a definite article. For example: The big dog = הכלב הגדול (hakélev hagadól). This rule is also applicable when we use more than one adjective for the same noun. The big beautiful dog = הכלב הגדול והיפה (hakélev hagadól ve'hayafé). Note that we say "big and beautiful". הגדול היפה is not as natural as "big beautiful" as in English. So now you can differentiate between: • הכלב גדול = The dog is big (ha'kelev gadol) • הכלב הגדול = The big dog (ha'kelev ha'gadol)
hope that helps.. behatzlakha!