He came into the bedroom and kissed her forehead with a mouth smelling of toothpaste.
I think that "a" is correct. Is "the mouth" possible? I think that he has only one mouth?
The mouth he has is big.
He has a big mouth.
Correct?
He came into the bedroom and kissed her forehead with a mouth smelling of toothpaste.
I think that "a" is correct. Is "the mouth" possible? I think that he has only one mouth?
The mouth he has is big.
He has a big mouth.
Correct?
When we are saying things like "I have a mouth", the indefinite article is perfectly correct. Using the indefinite article doesn't mean that we have more than one of something, it just acknowledges that there are many, of which we have one. Normally, we would refer to "my mouth" if we were referring to it in a way that distinguished it from others, but we do sometimes use the definite article, for example, "it hit me on the head", or "he kissed her on the lips".
In your specific example, using the indefinite article doesn't sound correct. Because it isn't just "a mouth" - it is "a mouth smelling of toothpaste". Because we use the definite article to determine something, if you said "the mouth smelling of toothpaste" it would sound you were saying he has a specific mouth that smells of toothpaste and perhaps some other mouths that smell of other things. It is similar to saying "I couldn't talk as I had a mouth full of food".