My friend who is not a native English speaker told me that one thing he struggled the most while learning English was the "a" and "an". He couldn't grasp the concept how it is possible that person knows which determiner to use before saying the word, until he learned it as just the part of the word and then he uses it depending on the context which one can "feel". So when he sees an apple, he says this is an apple.

Exactly what you mean by bigram.