What principle guides word combinations with "almost"?

I am trying to explain to non-native speakers how to use "almost." I can't formulate (a) rule(s) to follow with regard to nouns/pronouns. So far, my only ideas are that almost can be collocated only with words (or in situations) that describe measurement or comparison. However, even this seems to fail.

For example:

almost they = incorrect (no concept of measurement or comparison)

almost everyone = correct (measurement of individuals)

A platypus is almost a duck. = correct (comparison) They are almost the same. = correct (comparison) It is almost midnight. = correct (measurement of time vs. temporal adverbial) We are almost there. = correct (but spatial adverbial)

However:

He wrote almost a book. = incorrect, although in this case "book" would be a comparison or perhaps a measurement of the written material

I am pretty much at a loss. Thanks.

I don't know if there is a standard ESL guideline for this but we ask our team/colleagues to mentally substitute very nearly for almost and see if the sentence makes sense to them. If it does, almost can be used.

This isn't foolproof and probably won't make sense to native speakers, but it works in almost all the cases we encounter on a regular basis!

PS: This is a typical ESL challenge where learners swap most and almost. This is due to the influence of mother tongue on their sentence formation (native speakers usually won't have this issue and probably can't relate to why non-natives understand it this way). Another mother-tongue driven understanding that is common is that when learners say they have a doubt, they actually mean they have a question.