Sentences that begin with he are categorising sentences - that is, they say that something belongs in a particular category. For example "that is a fish" is a sentence that says that a particular thing can be put in the "fish" category. In te reo Māori, we would use a he sentence:
He ika tēnei.
This is a fish.
Literally, we could say "in the category if fish this is", though no one would ever say that in English.
There are three things that we can use in a sentence starting with he. Firstly, we can have a noun (such as "fish", "dog", "man"):
He tākuta ahau.
I am a doctor.
He kuri tērā..
That is a dog.
Seocndly, we can have an adjective ("good", "bad", "sad"):
He kino tēnei kurī.
This dog is bad.
He harikoa ahau.
I am happy.
THirdly, we can have both a noun and an adjective (a happy man, a noisy child, an angry policeman):
He kai pai te kūmara.
Kumara is good food.
He ara kino tēnei.
This is a bad road.
He tangata koa ia.
He's a happy chappy.
Each of these examples say that someone or something belongs in a particular category: "in the category of 'good food' is kumera", "in the category of 'dog' is Fido", "in the category of 'happy' I am".
If I see a long boat, I can either say:
He roa tēnei waka.
This boat is long.
Or:
He waka roa tēnei.
This is a long boat.
If I see a big rock, I can say several things:
He nui tēnei.
In the category of big this, near me is.
This is big.
He toka tēnei.
In the category of rock this, near me is.
This is a rock.
He nui tēnei toka.
In the category of big this rock, near me, is.
This rock is big.
He toka nui tēnei.
In the category of big rock this, near me is.
This is a big rock.
While this might sound complicated, it's excactly what we could say about a big rock in EnglishL this is big! this is a rock! This rock is big! This is a big rock!
If I see a yellow car, I can either say:
He waka kōwhai tērā.
That is a yellow car.
Or:
He kōwhai tērā waka.
That car is yellow.
He papa tākaro tēnei.
This (near me) is a playground.
He rākau tērā.
That (over there) is a tree.