Question

e.g. glad / Jane
You should pay attention to the a sound in the word glad.
Focus on the /ei/ sound in 'Jane'. The ei sound is a diphthong.
When there is a consonant followed by silent e at the end in one syllable word, generally the letter a is pronounced /ei/. For example. same, cake, skate, face, date, fate, etc. cf. fat, Sam

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Is my explanation grammatical?

Answer 1

Not really. The "a" sound in Jane is what is called a long vowel sound. It says the sound of the name of the vowel. "a" In using diacritical marks, a straight line would be placed over the vowel. The word "glad" does not have the long "a" sound it has the short "a" sound. The end of your instructions is correct. When a one syllable word ends in an "e" then the vowel in the middle will have a long sound. Example: fat - short, fate - long.