* An interjection: When "well" expresses surprise, agreement, or hesitation.
* An adverb: When "well" modifies a verb, adjective, or another adverb and introduces a new thought or clause.
Here are some examples:
Interjection:
* Well, I guess I could try that.
* Well, that's certainly interesting.
* Well, let's see what happens.
Adverb:
* Well, I'm not sure I agree with you.
* Well, the movie was pretty good, but the ending was a bit disappointing.
* Well, it's a long story.
When not to use a comma after "well":
* When "well" is part of a phrase: "She did well on the exam."
* When "well" modifies a noun: "He is a well-respected doctor."
* When "well" is used as an adjective: "The food was well-cooked."
In general, if "well" introduces a new thought or clause, it usually needs a comma. If it is part of a phrase or modifying a word directly, it doesn't.