Because duidelik is being used after the verb is. In that position, Afrikaans adjectives usually stay in their basic form.
So:
But before a noun, the adjective often takes -e:
This is a very common Afrikaans pattern.
Yes, is here works like English is. It is the present-tense form of wees (to be).
A useful thing to know is that Afrikaans does not change this form for different persons or for plural in the present tense:
So even in the plural, you still use is.
A rough English-friendly guide is DOY-duh-luk.
A few notes:
So DOY-duh-luk is only an approximation, but it is a useful starting point.
You can make those changes like this:
Notice two useful things:
So Afrikaans is quite simple here compared with English and many other languages.