What concept is a teacher addressing when emphasizing the long vowel sound in words like 'grain'?

The emphasis on long vowel sounds in words such as 'grain' relates to the alphabetic principle, which is the understanding that letters and combinations of letters represent the sounds of spoken language. In this case, the teacher is highlighting how the specific spelling of the vowel sounds contributes to the pronunciation and meaning of the word. By focusing on the long vowel sound, the teacher helps students make the connection between the written form of language and its corresponding sounds, which is crucial for decoding and reading proficiency. This knowledge enables students to read more effectively by recognizing patterns in how letters represent sounds in different words.