Words Starting With the Vowel "I"

As a volunteer English teacher to newly landed to-be citizens of Canada, please bear with me, as I am trying to be as specific as possible, without being overtly wordy.

The word of the day in Merriam-Webster’s daily post was "Itinerant."

Words that start with the letter "I" most often have a short, or informally, a soft vowel sound, if two consonants follow the letter "I." Many well used exceptions, such as if, in, and it.

Words such as intelligent, inconsolable, imbecile, irresistible, and most others follow this rule.

On the other hand, words such as itinerary, isolated, idol, ideal, and others give a long, or informally, a hard vowel sound.

Simply put, is there an English rule that could help a person newly learning the English language that would avail a more straightforward answer to this question? Thanks – Campaigner8