Cfxtrjtrk

Yes­ter­day I en­coun­tered this sen­tence (I’ll re­fer to the num­bered words in
my ques­tion be­low):

This is be­cause many stu­dents think that all of their sen­tences need to
be ‘com­plex’ (they don’t!) and¹ them² not un­der­stand­ing³ what a ‘com­plex’
sen­tence is.

I just can't un­der­stand the gram­mat­i­cal con­struc­tion af­ter the link­ing
word, and¹.

Why did the writ­er use the ‑ing form³ of the verb af­ter an ob­ject pro­noun
them²? And why did he start a sen­tence with an ob­ject pro­noun in the
first place?

Source: https://www.ieltsadvantage.com/2015/03/27/ielts-writing-complex-sentence/

This sen­tence:

This is be­cause many stu­dents think that all of their sen­tences
need to be com­plex and them not un­der­stand­ing what a com­plex sen­tence
is.

is apt to pro­voke gram­mat­i­cal con­fu­sion be­cause you aren’t ini­tial­ly
cer­tain about what two syn­tac­tic con­stituents (call them X and
Y) which the con­junc­tion and is co­or­di­nat­ing. The val­ue of
con­stituent Y fol­low­ing the con­junc­tion is al­ways the same (“them
not un­der­stand­ing what a com­plex sen­tence is”), but ex­act­ly what
X is varies in length. Is it (1) need X and Y, or is it (2)
think that X and Y, or is it (3) be­cause X and Y?

Here are those three pos­si­bil­i­ties:

1. This is be­cause many stu­dents think that all of their sen­tences
   need X and Y:
   
   This is be­cause many
   stu­dents think that all of their sen­tences need (X=to be
   com­plex) and (Y=them not un­der­stand­ing what a com­plex sen­tence
   is).




2. This is be­cause many stu­dents think that X and
   Y:
   
   This is be­cause many stu­dents think that
   (X=all of their sen­tences need to be com­plex) and (Y=them
   not un­der­stand­ing what a com­plex sen­tence is).




3. This is be­cause X and Y:
   
   This is
   be­cause (X=many stu­dents think that all of their sen­tences
   need to be com­plex) and (Y=them not un­der­stand­ing what a
   com­plex sen­tence is).

The clos­est to a cor­rect parse is the third one, which it takes your brain more
time to work out for sev­er­al rea­sons. One rea­son is be­cause it’s
the longest pos­si­ble val­ue for X, so your men­tal pars­er back­ing up
to find some­thing that makes sense hits the two short­er pos­si­bil­i­ties
first be­fore it comes up­on the cor­rect choice.

Another rea­son is be­cause of be­cause not ac­cept­ing a noun phrase
com­ple­ment with­out the prepo­si­tion of. So nev­er just “be­cause not
un­der­stand­ing”, on­ly ever “be­cause of not un­der­stand­ing”. (Cred­it to Janus for notic­ing this.)

The last rea­son is that the two con­stituents be­ing co­or­di­nat­ed here are not
quite the same thing: X is a fi­nite verb clause but Y is a
non-fi­nite verb clause. This is con­tribut­ing to your oth­er
con­fu­sion, since the sub­jects are no longer in the same case.
Fi­nite verbs have manda­to­ry sub­jects that when they’re pro­nouns
are in the sub­ject case (like I, he, them).

But the op­tion­al sub­jects of non-fi­nite verbs, when pro­nouns,
can­not be in the sub­ject case. They have to be in the ob­ject case
(like me, him, them) or in their pos­ses­sive de­ter­min­er forms (my,
his, them).

So that’s what’s hap­pen­ing here: them is the sub­ject of the gerund
clause head­ed by the non-fi­nite verb un­der­stand­ing.

It would have been more com­pas­sion­ate if the writ­er had rec­og­nizes
the po­ten­tial for con­fu­sion here and slight­ly re­struc­tured the
sen­tence to add more sen­tinels to help the read­er parse the sen­tence
more eas­i­ly.

Here are a few pos­si­bil­i­ties in that re­gard:

1. This is not on­ly be­cause many stu­dents think that all of their
   sen­tences need to be com­plex, but al­so be­cause of not un­der­stand­ing
   what a com­plex sen­tence is to start with.




2. This is be­cause many stu­dents think that all of their sen­tences
   need to be com­plex and be­cause they don’t un­der­stand what a com­plex
   sen­tence is in the first place.




3. This is be­cause of many stu­dents think­ing all their sen­tences
   need to be com­plex and not un­der­stand­ing what a com­plex sen­tence
   is.

Those should all be eas­i­er on the read­er than the orig­i­nal.
Once you un­der­stand what was changed in each of them to fa­cil­i­tate
com­pre­hen­sion, you will be­gin to see where you foundered in the un­changed
sen­tence you first pre­sent­ed.

In a comment, BillJ wrote:

... and them not understanding what a
‘complex’ sentence is.

There is a structural problem with your sentence (as others have pointed out) but leaving that aside, the answer to your question is that non-finite gerund-participial clauses take accusative and genitive subjects. In your example, the pronoun is subject of the non-finite clause "them not understanding what a complex sentence is", so the subject could be either accusative "them" or genitive "their".

The overall structure of your example is: [S this is because S ], by which I mean that the whole thing, [S ... ], is a sentence S, and within that S is another S after "because". That S after "because" is [S many stu­dents think that all of their sen­tences need to be com­plex and them not un­der­stand­ing what a com­plex sen­tence is ].

Working our way down in the structure, this S seems to begins with an S "many students think that all of their sentences need to be complex" followed by "and". If so, we can appeal to a general principle about "and" to identify the structure of the rest. That principle is that "and" (together with other coordinate conjunctions) is preceded and followed by phrases of the same type, and that the entire phrase formed is itself a phrase of that very same type.

It follows that "them not understanding what a complex sentence is" must be an S, but that is a problem here, because what precedes the "and" is a finite (tensed) clause "all of their sentences need to be complex", while what we have here is not like that. As it stands the example appears to be unacceptable.

One way to repair the grammatical problem is to substitute the related finite clause [S they don't understand what a complex sentence is ]. Then, we'd have:

This is be­cause many stu­dents think that all of their sen­tences need to be com­plex, and they don't un­der­stand­ what a com­plex sen­tence is.

I think there's only one way to parse the sentence, as it is, as a grammatical one:

A coordination of these two clauses:

This is because many students think that all of their sentences need to be ‘complex’ (they don’t!)

and

This is them not understanding what a ‘complex’ sentence is.

If this was intended, the sentence is awkward at best, though, in part because the them not understanding what a ‘complex’ sentence is is too far from This is.

So, I'd like to parse it as a supplementation (instead of a coordination) by adding the necessary punctuation such as a comma or a dash:

This is because many students think that all of their sentences need to be ‘complex’ (they don’t!), and them not understanding what a ‘complex’ sentence is.

And it's better to omit the 'and' as follows:

This is because many students think that all of their sentences need to be ‘complex’ (they don’t!), them not understanding what a ‘complex’ sentence is.

In a comment, Jason Bassford wrote:

It's an example of a lack of parallelism. Stylistically, both verbs should take the same form. ("students think and don't understand" or "students thinking and not understanding"). It's not actually an error as it is, but it would sound better if it were rephrased.