Meaning: The monkey moved forward and backward on the vine.
Choice of Words: Monkey is the subject, swung is the verb, back and forth is the adverb describing the direction of the swinging.
Alternative Expressions
The monkey moved to and fro.
Meaning: This means the same as the first sentence, that the monkey was moving back and forth.
Choice of Words:Monkey refers to the animal. Moved is a general verb that describes any kind of movement. To and fro are old-fashioned words that mean back and forth.
The monkey swayed back and forth.
Meaning: This means the monkey was moving back and forth in a gentle way.
Choice of Words:Monkey refers to the animal. Swayed means to move gently back and forth. Back and forth indicate the two directions of the movement.