1. He met the boy here.

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Q1: Does 'the boy' modify 'met' or 'the boy'?
Q2: Is 'here' an adverb or an adjective?
Q3: Did he meet the boy here for the first time? Or did he meet the boy here whom he knew before?

Q1 -- "the boy" is a noun and its article; it doesn't modify anything.

Q2 -- "here" is an adverb.
"Adverbs answer one of these four questions: How? When? Where? and Why?" http://www.chompchomp.com/terms/adverb.htm

Q3 -- There's no way to answer this without the rest of the paragraph to understand the context.

1. He met the boy here.

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Q1: Does 'here' modify 'met' or 'the boy'?
Q2: Is 'here' an adverb or an adjective?
Q3: Did he meet the boy here for the first time? Or did he meet the boy here whom he knew before?

Q1 and Q2 -- "here" is an adverb. It tells where.

"Adverbs answer one of these four questions: How? When? Where? and Why?" http://www.chompchomp.com/terms/adverb.htm

Q3 -- There's no way to answer this without the rest of the paragraph to understand the context.

Thank you very much for your help.

What about the following?
1. Look at the boy there.
2. He knows the boy there.
3. He saw the boy there.

Does 'there' modify 'the verb' or ' the noun phrase'?

I know you want "there" to be an adjective modifying "boy" even though the standard use for "there" is as an adverb. It tells where.

https://www.merriam-webster.com/dictionary/there
Scroll down to the section there adjective
Notice that it says that using "there" like this is for emphasis or is non-standard. In your sentences, the word seems to be used for emphasis.

To answer these questions, we need to look at the function and meaning of each word in the sentence.

1. Does 'the boy' modify 'met' or 'the boy'?
To determine this, we need to identify the subject and object of the sentence. The subject is 'he', and the verb is 'met'. 'The boy' is the direct object, as it is the recipient of the verb 'met'. So, in this sentence, 'the boy' is the direct object and not modifying anything.

2. Is 'here' an adverb or an adjective?
To determine if 'here' is an adverb or an adjective, we need to consider how it is used in the sentence. Here, 'here' is describing the location where the meeting took place, indicating that it functions as an adverb modifying the verb 'met'. It answers the question "where did he meet the boy?"

3. Did he meet the boy here for the first time? Or did he meet the boy here whom he knew before?
The sentence given does not provide enough context to determine if this is the first meeting or if there was prior knowledge of the boy. Additional information would be needed to answer this question.

In summary:
- 'The boy' is the direct object and not modifying anything.
- 'Here' is an adverb, modifying the verb 'met'.
- The sentence does not provide enough context to answer whether it was the first meeting or someone previously known.