Because een means one, while 'n is the indefinite article a/an.
So:
That means:
In everyday speech, nog 'n appel hê is very common. The version with een sounds a bit more specific or emphatic.
Yes. wil is the present-tense form used for want.
One helpful thing for English speakers is that Afrikaans verbs do not change much for different subjects in the present tense:
So you do not have to learn lots of different endings the way you do in some other languages.
Hê is pronounced roughly like heh with a longer vowel, or a bit like the vowel in British air without a strong r sound.
The ê is important. In Afrikaans, this accent mark helps show the vowel quality and length. So hê is not written the same way as plain he would be.
A rough pronunciation guide:
It is best to think of it as a long, open e sound.
It can be understood perfectly well, but it may sound a bit direct depending on the situation.
If you want to sound softer or more polite, Afrikaans often uses forms like:
So Ek wil nog een appel hê is grammatically fine, but in real conversation a learner may often hear graag or a question form instead.
People may still understand you, but the full and more standard structure is Ek wil nog een appel hê.
Including hê makes the sentence feel complete, because Afrikaans often expresses this idea as want to have rather than just want + object.
So for learners, it is safest to keep hê in this kind of sentence.