A question about 'must have been someone'

Question

I am recently struggling with the grammar MUST HAVE BEEN.

My question is can i say

"It must have been someone stealing/ stole / who stole my wallet "

I think who stole is right, but might also be stealing, because one sentence can not have two verbs ? so the second one has to be turned into ING form.

can anyone tell me which on is correct and the grammar behind this ?

Much appreciated !

Answer 1

Old (Simple) Answer

"Stealing" and "who stole" my wallet both work, although they have different meanings.

If you're talking about someone who stole your wallet, you'd refer to them with "It must have been someone who stole my wallet".

If you're talking about the event in which your wallet was stolen, it would be "It must have been someone stealing my wallet".

If you're trying to say that the reason your wallet is missing is that it was stolen, you might consider "Someone must have stolen my wallet" or "It must have been that someone stole my wallet".

Incidentally, "have been" followed by the gerund form ("ing" for most verbs) puts the verb in the "Perfect Indicative" form.

New (Overly Complicated) Answer

I have updated this to address some of the comments/potential cases that might come up in the real world.

Sentences can have multiple verbs in them, actually (remember, the sentence is everything from the first word to a period).

An example

In this specific case, it's probably helpful for me to mention that you can build up phrases that act the same way as a noun would.

For example, take the simple sentence

"I want a hamburger". 

"I" is a noun (the subject of the sentence), "want" is the verb, and "hamburger" is a noun (the direct object).

If I want to be more specific, I can build up a phrase using nouns, verbs, adjectives, etc. that function just like a noun would. Really, in the above example, you already have a noun phrase "a hamburger" is already a noun phrase, consisting of the article "a" and the noun "hamburger". These are all sentences where "hamburger" is a noun that functions as part of a slightly larger noun phrase:

         I                  want                  a hamburger
   [noun, subject]   [verb, transitive]  [noun phrase, direct object]

         I                  want                 the hamburger
   [noun, subject]   [verb, transitive]  [noun phrase, direct object]

         I                  want                  hamburger
   [noun, subject]   [verb, transitive]    [noun, direct object]

(Individual nouns work as noun phrases, too. I could have written "noun phrase" beneath the word "I" in each example and the individual word "hamburger", I just didn't)

I can build this up into something more complicated, too:

     I                  want             a hamburger with tomatoes and lettuce that has been covered with mayonnaise
[noun, subject]   [verb, transitive]                 [noun phrase, direct object]

If you /really/ want to dive deep into it, I find this diagram helpful, but if it confuses you just ignore it- it's not something the average person would know about (although if you're curious, this is an example of a "syntax tree" and you can probably find examples of it in your native language) https://media.geeksforgeeks.org/wp-content/uploads/20200330000517/Screenshot-2020-03-30-at-12.02.08-AM.png

Back to your question

Your question is slightly complicated. First off, I can tell you that "It must have been someone stole my wallet" is going to be considered incorrect in formal writing.

First and foremost, I want to explain "must"

Must

Must is what's called a modal auxiliary verb. Don't dig too far into the specifics of this other than "can, could, may, might, shall, should, will, would, and must" can't be used without some other verb (whether you actually use it or whether it's implied. You could respond to a child asking you "May I play video games?" with "You may "). The history behind these words is complicated and weird (shall and should are related historically, for example, but they mean different things to most speakers), so it's better just to look at a list of ways people use each one.

"Must" is used to express certainty (or near-certainty) or requirement. If I say "It must have been x", I'm using it in the following way:

• If x is a person, action, or thing, I could be saying "I'm pretty certain that it was that person/action/thing that [insert conversation topic here]"

To shine some light on the rest, let me lay out the possible meanings here explicitly:

It must have been someone stealing my wallet

• "It must have been someone stealing my wallet" can be viewed two ways (since "someone stealing my wallet" could be referring to the person doing it, or to the action itself):
  1. You are putting emphasis on the person. Here, "it" is either a dummy/an impersonal pronoun. "I wonder who that was? Oh, my wallet is gone! It must have been someone stealing my wallet")
  2. You are putting emphasis on the action. Here, "it" is either a dummy pronoun or referring to the event of your wallet being stolen. "Why is my lock destroyed? Oh no! It must have been someone stealing my wallet"

It must have been someone stole my wallet

I want to emphasize that this is a niche use that you would usually only see informally/with certain people. I would never use this construction, personally.

You could possibly use this to put an emphasis on the action in the past tense "I'm recalling details about my day... Wait, why was my lock missing? Oh no! It must have been <that/due to the fact that> someone stole my wallet, then put it back"- notice the "put it back" part, though. If this didn't happen, you'd just say "It must be <that/due to the fact that> someone stole my wallet" (and you could use this in any case).

You'd usually include the word "that" anyway (I don't have a good reason for this offhand other than that dropping the word "that" sounds weird), but I wouldn't be surprised if it got dropped in informal speech/writing sometimes. I would not, however. I also want to say that it's the only one of the three that my grammar-checker is yelling at me about.

In the event that you were referring to a person here, you need to say "It must have been someone that stole my wallet" or "It must have been someone who stole my wallet". There might be certain dialectical exceptions to this, but it wouldn't be standard writing.

It must have been someone who stole my wallet

• "It must have been someone who stole my wallet" can be viewed in one way (it can only refer to the person):
  1. You are putting emphasis on the person. Here, "it" is either a dummy/an impersonal pronoun. "I wonder who that was? Oh, my wallet is gone! It must have been someone who stole my wallet")

Answer 2

what confuses me is the difference between MUST HAVE BEEN SOMEONE DOING and SOMEONE WHO DID

Part of your issue has to do with "someone". Someone complicates the situation. Let's use real creatures for the time being, and then you can ask more about "someone" in a separate followup question.

who likes to have personal noun or pronoun as antecedent, although sometimes people will use "who" with nameless animals and maybe even bumblebees, but that is not the general rule:

It must have been a bee that stung you.

A parent might say to a small child:

It must have been a bumblebee who stung you.

If the new shoots on the plants in the garden are gone:

A rabbit must have been nibbling on them.

It must have been a rabbit nibbling on them.

It must have been a rabbit that was nibbling on them.

A rabbit must have nibbled on them.

It must have been a rabbit that nibbled on them.

Peter Rabbit must have been nibbling on them.

It must have been Peter Rabbit who was nibbling on them.

It must have been Peter Rabbit who nibbled on them.

My car keys are gone. It must have been Jane who took them.

It must have been my sister who took them.

ODD, REQUIRING ADDITIONAL CONTEXT:

It must have been my sister who was taking them.

In order for the sentence above to be cogent, the keys must have been missing on more than one occasion.