Over the last couple of days I've started noticing sentences like this one in Grace's speech:
"Mine not know this goes"
The sentence above translates into standard English as
"I don't know [where/how] this goes."
The context is putting a puzzle together, so it's pretty clear that this is what she means.
Null complementizers are of course allowed in English in sentences in alternation with "that" (I saw the man you mentioned / I saw that man that you mentioned, etc.). However, Grace is using a null complentizer where we would use an unstressed question word as a complementizer -- where, when, how, etc. She knows these question words in other contexts, but question words as complementizers play a different grammatical function and are harder to pick out, I'm sure, since they're unstressed, so it makes sense that she hasn't picked them up here yet.
I wonder how comprehensible someone would be if they left out all of the question word complementizers... evidence so far is that it wouldn't be too bad -- when I first mentioned the sentence above to Katharine, she could swear she'd heard a "how", though when we got Grace to repeat it it was clear there was no "how" there. It's amazing how much filling-in we do when we listen, and how much of it is unconscious. Even though I'm aware of lots of the irregularities of Grace's speech (using a "t" for a "k", for example), in every day interactions it feels like she says words like "kitchen", "cook", etc. completely normally (until, that is, I misunderstand her, at which point I realize that for a "t" sound I have to guess a wide range of possible letters -- t, k, qu, etc.).