You may be right. The thing is in Arabic, one word can have multiple meaning in Arabic. So, if you are speaking to an Arab, they will get what you mean. Now to translate the below sentence:
Duo is fun=>دو ممتع/مسلى
ممتع=fun Interesting=مسلى (musali)
Hope I didn't confuse you more.
Thanks Scruffy-bear for your explanation of the mnemonic. I thought you were complementing Vismaior under his comment and did not realize that you were answering my question directly under my comment. I deleted my question since I thought it was no longer necessary. I didn't realize you were answering me directly and that deleting my question would delete your explanation of the mnemonic. Please add your comment back.
I'm guessing in this sentence, Duo is considered masculine, which is why مُمتِع is in the masculine form, rather than the feminine form? https://en.wiktionary.org/wiki/%D9%85%D9%85%D8%AA%D8%B9