What is a guarding Aleph? Unterstanding its role in كَتَبُوا

Have you ever thought why we spell كَتَبُوا (they wrote) with an Aleph at the end?

When a conjugated verb has a و at the end, and when this و is preceded by ضَمّةٌ or by فَتْحةٌ, then the ا is often used (and written), particularly in the plural of verbs.

Let’s see some examples:

So, why is it there? In short:

The Aleph protects it against the possibility that the و could be separated from the body of the word to which it belongs – and thus could be mistaken for the word “and” (i.e., the conjunction وَ).

For this reason, this Aleph is called أَلِفُ الْوِقايةِ, the guarding Aleph, or الْأَلِفُ الْفاصِلةُ, the separating Aleph.

Note that there are other explanations as well, but most of them share the idea of guarding and protecting the و from being mistaken for other letters or from being absorbed.