He never talked to Sanena, and does not like her either.
That comma is the least of your worries with this particular
sentence. The real problem is that and is a terribly
polysemous conjunction, one carrying too many possible
meanings for a single punctuation mark alone ever to disambiguate
completely and safely.
First and foremost, this and seems to join the two verb phrases never
talked to Sanena with does not like her either. Subliminally, it
also occupies the grammatical role of the subject and thus joins
bother both subjects of the two respective clauses.
Here’s a constituent tree of your sentence, courtesy of the CMU
(S (NP He)
(VP (VP (ADVP never)
(VP does not
Those subjects happen to represent the very same subject here,
but conceivably might be an additional subject elsewhere:
- He went, and John followed.
- He went and was followed by John.
This implicitly binds the object as well:
- He went, and John followed, to the destination.
However, this functionality is not used when the object is
reiterated in the second clause.
More importantly, either cannot be used as a predicative
adverb in the second clause; that would instead need to be
neither as in
- .. and neither likes her.
The preference against that leads to the suspicion that either might
be a sentence adverb:
- ...and either does not like her.
This obviously sounds as if it should be followed by or:
- ...and either dislikes her, or at least does not care.
This is semantically close to but ... anyway.
The real kicker is that either as a terminal sentence adverb
might also apply to the first clause to confer a symmetric
relation. Obviously Sienna never talked to him, either. The
corollary implied by analogy is that Sienna does not like
him, although the perspective of the sentence does not allow
such inference. Rather it implies the question for the fact.
It would not be unusual to follow up by stating an asymmetry:
- But Sienna was secretly in love with him.
Another implication is that of how he could know not to like
Sienna. If either binds both clauses symmetrically, he either
does not yet like her because he has not had the chance, or else
he does not talk to her because he does not like her. This state
of mutual exclusion is the primary function of the either..or
and neither..nor disjuntive constructions, which might be
the reason that neither too nor also has been used here.
As such, it makes sense to set off the second clause with a comma,
as adposition, and to set off the adverb from the second clause.
Otherwise either would serve no purpose at all because leaving
it out would retain the main conjunctive force resting on the