Practical English Usage by Michael Swan has a good, brief explanation of participles. S1: John met Mary [ walking down the street. ] ( no comma before walking ) = John met Mary who was walking down the street. S2: John met Mary [ ,walking down the street. ] ( comma before walking ) = John met Mary while John was walking down the street. This is normally considered an error (misrelated participles). S3: Walking down the street, John met Mary ( comma before John ) = While John was walking down the street, John met Mary. This is the preferred form of S2. Q1: S1 and S3 have different meanings. S2 is a bad version of S3. Q2: In S1, "walking down the street" is a participial clause which functions as an adjective modifying Mary. In S3, "Walking down the street" is a participial clause which functions as an adverb modifying "met." Q3: In S1, Mary was walking. In S2 and S3, John was walking.