"The fences are long."
Translation:Die Zäune sind lang.
What now? Why isn't Der Acceptable?
Because die Zäune is the subject of the verb sind, so it has to be in the nominative case.
So you need the plural nominative article die.
der would be plural genitive (wrong case), masculine nominative (wrong number - singular instead of plural), or feminine genitive/dative (wrong gender and case).
If you're on a mobile device, try long-pressing the a o u s keys to get access to modified versions such as á à â ä å ..., ö ô ø ..., ú ù ü ..., š ß ....
If you're on a computer (Windows, Mac, Linux), you (a) should have little buttons underneath the input field letting you insert ä ö ü ß, (b) may wish to install a German keyboard layout.