No. Afrikaans does not have grammatical gender in the way many European languages do.
That means nouns are not classified as masculine, feminine, or neuter for article purposes. You do not need to learn different forms of the depending on the noun.
So fase is just a noun, and it takes die like other definite nouns.
Yes. Derde is the ordinal number meaning third.
Afrikaans ordinal numbers are often formed with -de or -ste, but some are irregular enough that it is best to learn them individually. Common ones are:
So drie means three, while derde means third.
Because cardinal numbers and ordinal numbers are different forms.
Just as English has:
Afrikaans has:
So derde is not unusual; it is simply the correct ordinal form.
Afrikaans verbs are much simpler than English verbs. In the present tense, the verb usually does not change according to the subject.
For the verb wees (to be), you get:
So is stays the same. This is one of the easier parts of Afrikaans grammar for English speakers.
A simple learner-friendly approximation is:
dit is dee DARE-duh FAH-suh
A few pronunciation notes:
The biggest surprise for many English speakers is that die is pronounced dee.
This sentence uses the basic Afrikaans main-clause word order:
Subject + verb + the rest
So:
That is very similar to English.
Afrikaans does have some word-order rules that differ from English in longer sentences, especially when another element comes first, but this sentence is straightforward and basic.
Yes, but it changes the nuance a little.
Hierdie is more explicitly this.
Dit is more neutral and extremely common.
So the original sentence is perfectly natural, and probably the most basic way to say it.