Here's why:
* "Ali and Grigorio met" is a complete verb phrase, showing an action.
* "to pick berries" is an infinitive phrase, acting as a purpose or reason.
* "in nearby field" is a prepositional phrase modifying "berries."
To determine the part of speech, we need to know what word is missing. For example:
* "Ali and Grigorio met *there* to pick berries in nearby field." "*There*" would be an adverb.
* "Ali and Grigorio met *to pick* berries in nearby field." "*To pick*" would be an infinitive acting as a verb.
* "Ali and Grigorio met *at* the nearby field to pick berries." "*At*" would be a preposition.
Please provide the complete sentence so I can accurately identify the part of speech.