Go to Almaany and then to the Arabic-Arabic dictionary (see print screen). Or go directly to the Arabic-Arabic dictionary. Enter the word whose plural you want to look up, for example مِفْتَاحٌ and click on 'ابحث' (search).

Find the right result and look behind الجمع. The word مفتاح therefore has two plural forms: مَفاتِيحُ, مَفَاتِيحُ



If you enter the plural, you will get the explanation جمع مِفْتَاح, this means the plural of مِفْتَاحٌ. So pay close attention to the difference between الجمع (the plural) and جَمْعُ 'word in the singular' (The plural of 'word in the singular').