Present participle as a modifier

Suppose A and B are friends and B has only one brother.

During a conversation between A and B:

A: you know, yesterday I met your brother at the supermarket buying some stuff for his children.

Can we use a present participle(buying) in this way. I mean to say I know when a present participle is used this way, it normally modifies the subject of the preceding clause. But, can it modify the object of a verb or maybe any other noun in that clause or maybe a smaller clause inside the main clause?

Consider the second sentence please:

Yesterday I met your brother walking to work.

How are the first sentence and this sentence different?

Explanations concerning this question can be found in A Comprehensive Grammar of the English Language, 1985 edition. The subjects concerned are postmodification by -ing participial clauses and supplementive clauses in final position.

17.28 Postmodification by -ing participle clauses

Postmodification of the noun phrase is possible with all three of the nonfinite clause types: -ing participle, -ed participle, and infinitive clauses. The correspondence between -ing clauses and relative clauses is limited to those relative clauses in which the relative pronoun is subject:

[1] The person {who [will write/will be writing/writes/is writing/wrote/was writing] reports} is my colleague. [1a] The person writing reports is my colleague.

The nonfinite clause writing reports in [1a] may be interpreted, according to the context, as equivalent to one of the more explicit versions in [1]. Other examples of postmodifying -ing clauses:

♦ The dog barking next door sounded like a terrier. ['which was barking next door']
♦ A tile falling from a roof shattered into fragments at his feet. ['which fell from a roof']
♦ You should look for a man carrying a large umbrella. ['who will be carrying a large umbrella']

It must be emphasized that -ing forms in postmodifying clauses should not be seen as abbreviated progressive forms in relative clauses. Stative verbs, for instance, which cannot have the progressive in the finite verb phrase, can appear in participial form (cf 4.4, 14.19):

♦ [2] This is a liquid with a taste resembling that of soapy water.
['which resembles'; not: '*which is resembling']
♦ [3] It was a mixture consisting of oil and vinegar. ['that consisted of'; not: '*that was consisting of']

In all instances, the antecedent head corresponds to the implicit subject of the nonfinite clause. There is no nonfinite postmodifier, therefore, corresponding directly to the relative clause in [1b], without recourse to the passive [1c]:

♦ [1b] Reports that my colleague is writing will be discussed tomorrow. ♦ [1c] Reports being written by my colleague will be discussed tomorrow.

There are sharp constraints upon aspect expression in the participle clauses used in postmodification. We have just noted that resembling in [2] (a taste resembling that of soapy water) obviously could not represent the progressive, and the neutralization of the aspectual contrast can further be seen in [4] in contrast with [4a]:

♦ [4] the man {who works/who is working} behind the desk
♦ [4a] *the man {*working/being working} behind the desk

On the loss of this aspectual distinction in nonfinite verb phrases, cf 3.56. Similarly the perfective aspect cannot usually be expressed in the nonfinite clause. Compare [5] and [5a]:

♦ [5] The man who won the race is my brother.
♦ [5a] ?*The man having won the race is my brother.

However, in a structure with an indefinite noun phrase as head, as in [6], perfective aspect is more acceptable:

♦ ?Any person or person having witnessed the attack is under suspicion.

The tense to be attributed to the -ing clause will usually be that of the finite clause in which the noun phrase occurs, especially if the noun phrase is object:

♦ Do you know the mnan talking to my sister? ['who is talking to my sister']
♦ Did you know the man talking to my sister? ['who was talking to my sister']

The tense of the nonfinite clause can also be inferred from the context:

♦ [7] The man sitting next to her (now) was speaking on the radio (last night). ['who is sitting']

In a sentence like [7], the tense of the -ing clause would be assumed to be the present tense [8], unless the context suggests otherwise, as in [8a]:

♦ [8] the man sitting next to her ['who is sitting']
♦ [8a]the man sitting next to her on that occasion ['who was sitting']

In [9], the past tense was indicates the tense of being questioned and does not mean that he was no longer my brother, ie [9] = [9a]:

♦ [9] The man being questioned by the police was my brother.
♦ [9a] The man *that was (being) questioned by the police is my brother.

_{Note: In some cases, -ing participles occur in frozen expressions with no relative clause alternative, eg: for the time being (not: '*for the time that it is')}

¹ _{* : unacceptable
    ? : native speakers unsure about acceptability
  ?* : tending towards acceptability but not fully acceptable}

There can be ambiguity in the postmodification by means of an -ing participial clause.

15.62 Supplementive clauses in final position
In spite of their resemblance to nonrestrictive relative clauses, supplementive clauses need not be separated from their matrix clause intonationally when they occur in final position. The following are therefore alternative renderings of the same sentence, differing only in that [1] has two focuses of information, whereas [2] has only one:
♦[1] The manager apPRÒACHED us, SMÌLing.
♦ [2] The manager approached us SMÌLing.

One result of the alternative shown in [2] is the possible neutralization of the formal difference between nonfinite clauses functioning as supplementive clauses and those functioning as complementation of the verb. Thus [3] is ambiguous:

♦ [3] I saw Pam going home.

On one interpretation (that of the supplementive clause), I is the implied subject of going home, whereas on the other (that of verb complementation), Pam is the overt subject.
Another result of the lack of intonation is illustrated in [4] and [5]. When the -ing participle immediately follows certain finite verbs with existential meaning, the latter seem close to being aspectual catenatives (cf 3.49):

♦ Frank sat reading the newspaper.
♦Edith came running towards us.

Further, a sentence such as [6] is ambiguous in more than one way:
♦ [6] I caught the boy waiting for my daughter.

In addition to the two possible structures of [3], this may be interpreted as having a third structure, in which the nonfinite clause is a postmodifying clause:
♦ 'I caught the boy while I was waiting for my daughter.' [supplementive clause]
♦ 'I caught the boy in the act of waiting for my daughter.'[verb complementation]
♦ 'I caught tl~? boy who was waiting far my daughter.' [postmodification]

From what can be gathered in the two sections shown above (the first only is complete), it follows that the first sentence is correct and carries no ambiguity whereas the last one is ambiguous. There is no ambiguity in the first one as the determiner "his" makes clear that the implicit subject of "buying" is "brother".
The second is ambiguous as it has the two following interpretations.

• Yesterday I met your brother while I was walking to work. [supplementive clause]
• Yesterday I met your brother, who was walking to work. [postmodification]

Notice that a comma is necessary in the corresponding formulation, which is not found in 17.28 [1]. The reason is that in this latter case the postmodification is defining whereas in "Yesterday I met…" it must be descriptive.
There is no possibility of verb complementation as "meet" does not possess this construction.