Because hê is the infinitive form, meaning to have, while het is the finite form used in normal present-tense sentences.
Compare:
So after the subject in a normal statement, you use het, not hê.
Because tyd here is being used as an uncountable noun, just like time in English.
You say:
Likewise in Afrikaans:
If you were talking about a specific unit or occasion, the wording would be different.
They are close, but not always identical.
Ekstra tyd often suggests something added beyond what was expected or planned.
Examples:
So in many situations they can overlap, but ekstra emphasizes additional time.
Because this is a simple main clause, and Afrikaans often uses the same basic order as English in that kind of sentence:
So:
However, Afrikaans word order changes in other situations, especially in questions and subordinate clauses.
For example:
That final het in the subordinate clause is a very important Afrikaans pattern.
You usually make a yes/no question by inverting the subject and verb:
Notice that Afrikaans does not need a separate do the way English does.
It can be plural, but not in this sentence.
In Ons het ekstra tyd, the singular form is natural because time is being treated as an uncountable amount.
You would use tye in contexts like:
So here, tyd is definitely the right form.