"The trees have apples."
Translation:Die Bäume haben Äpfel.
Exactly! Both the English and the German phrase are very awkward and not used in everyday speech!
The forms of German verbs change depending on the subject. You can see remnants of this in English, e.g. "I have", but "he has".
haben (to have)
ich habe (I have)
du hast (you [singular informal] have)
er/sie/es hat (he/she/it has)
wir haben (we have)
ihr habt (you [plural informal] have)
sie/Sie haben (they/you [formal] have)
"habt" is used with "you" [plural informal]. "Die Bäume" (the trees) can't be replaced with "you", but they can be replaced with "they". Therefore you have to use "haben" and not "habt".
Having said that, the whole sentence sounds a bit odd to me.