The particles that carry meaning

An ism can stand alone and so can a fi'l. A harf cannot. مِنْ on its own says nothing; مِنَ الْبَيْتِ says a great deal. That is why the classical definition describes it as a word whose meaning appears only in something else.

There are not many of them — under a hundred in the whole language — and they carry an enormous share of the work. Learn the particles and long stretches of the Quran open up.

One question sorts the whole section. Does the particle change the ending of what follows? If it does it is 'āmil: مِنَ الْبَيْتِ, and بَيْت is now in jarr. If it does not it is ghayr 'āmil: زَيْدٌ وَعَمْرٌو, where the wāw joins but changes nothing by itself.

A few letters refuse to sit on one side — وَ, لَ, مَا, لَا each do several different jobs, and those have a page of their own at the end of the section.

If a harf means nothing on its own, how can it be a word?

Because meaning is not the same as standing alone. A harf carries a meaning that only appears once it is placed beside something — and remove it and the sentence collapses. It is a word by what it does, not by what it can say by itself.