Verb form to finish someone else's sentence?

Question

In spoken language, it sometimes happens that one person starts a sentence and another speaker finishes it, interrupting the first speaker. (This may be impolite in some situations, but not always).

A: I ...
B: ... clean the apartment today?

The verb clean doesn't change in first and second person singular.

In German, it would change:

A: Ich...
B: mache / machst heute die Wohnung sauber?

Should B use mache to keep the predicate congruent with the subject or should B use machst to keep the meaning intact (i.e. A is the one who does something)?

I understand this is not really a question of grammaticality (since the subject changes mid-sentence), but of the usage of spoken language.

Answer 1

The subject of the sentence does not change grammar-wise. Therefore the following is preferable:

A: Ich...

B: mache heute die Wohnung sauber?

"Ich" not being the actual speaker when continuing the sentence has to be analysed on the semantic level and does not concern grammar.

If, however, the other person is rather posing a question than completing the sentence, I could imagine hearing the other version with the "du" implicitly there and omitted as part of colloquial language. As a non-native speaker, though, this would almost always sound odd.