The vowel sound in "grain" is a long "a" sound, similar to the vowel sound in "say" or "day."
Here's how you can tell:
* Spelling: The "ai" combination in "grain" usually indicates a long vowel sound.
* Pronunciation: When you pronounce "grain," you hold the "a" sound for a noticeable amount of time.
Short vowel sounds are typically found in words where the vowel is followed by a consonant, like "cat," "dog," and "sun."