There are words that, when pronounced with an British English accent, appear to contain the letter "R", but which are not written as containing one.
Bath and path, for example.
In contrast words like Hearth, and Dearth, which contain the same vowel + "TH" pattern, but also contain an "R".
Are there any available resources that can be used to explain the origins of this, how to properly teach where it applies, or to explain how a student might identify where this is the case.