"I am watching you, not her."
Translation:Én téged nézlek, nem őt.
That part is correct, "látlak" and "szeretlek" clearly identify both the subject and the object. But they do not emphasize either on their own.
If you want to emphasize the subject, you must not omit the subject.
If you want to emphasize the object, you must not omit the object. The verb does not substitute for either, it just implies them.
Olvasok - I read
ÉN olvasok - It is I who reads
Látlak - I see you
Látlak téged - I see you - "téged" is not necessary
Téged látlak - I see YOU - It is you that I see
Nem látlak - I don't see you
Nem látlak téged - I don't see you - "téged" is not necessary
Nem téged látlak - It is not you that I see
Téged nem látlak - (I may see everyone else but) you I do not see
In these last two sentences, if you omit "téged", how do you know what is negated?
Nézlek - I am watching you.
Nézlek téged - I am watching you - "téged" is not necessary
Téged nézlek - It is YOU that I am watching
Téged nézlek, nem őt - I am watching YOU, not HER.
You get the point.
Now, but "Én" can be omitted, as the whole sentence is about "me", and there is not necessarily an emphasis on it.
And the second verb was omitted, as it is a repetition, even though in a different person. The whole sentence would be like this:
"Én téged nézlek, nem őt nézem."
The second verb almost feels wrong if it is not omitted. The second repeating verb is usually omitted. And it does not need to match in person or number. Only it has to be the same verb.
"Nem én olvasok, hanem ők (olvasnak)".
BTW, another question here, on the English sentence. Who is not watching whom? I am talking about "her".
Is it me not watching her:
"Én téged nézlek, nem őt",
or, is it her/she not watching you:
"Én nézlek téged, nem ő".?
Or would you say this latter one differently, maybe to avoid confusion?
"I am watching you, not she"
"I am watching you, she is not"
Any thoughts on this?
To put Én nézlek téged, nem ő into English without confusion, the cleanest solution might be to turn it around: "She isn't watching you, I am."
"I am watching you, not she" runs into a real tangle of problems. Among other things: Despite the fact that "she" cannot possibly be an object, the sentence still sounds more like it means Én téged nézlek, nem őt".
Basically, I would say, English speakers can sometimes have a very hard time hearing something in the final position of a sentence as a subject. It's easier to hear this "I am watching you, not she" sentence as meaning "I am watching you, not her" - but spoken with a grammatical error - than it is to hear it for what it actually means. Mizinamo is accurate when he says that the "correct" version sounds stilted and so we mentally "correct" it to the simplest possibility that sounds like something normal.
This problem is alleviated by putting a verb after the pronoun - then the subject really sounds like a subject! That's why my solution "She isn't watching you, I am" works well.
So I would also say that your sentence, "I am watching you, she is not" is also both correct and will be heard and understood correctly. For me - and I can't explain why - it does not put as much emphasis on contrast (Not she but I) as the version I offered. It is more like two loosely connected statements. But this can be changed by the use of stress when speaking.
In modern colloquial English, for many speakers, it can be either.
In conservative English, her must be accusative, and it can only mean "I am watching you; I am not watching her". The other version would have to be I am watching you, not she (which for some speakers today would sound stilted).
I am watching you, she isn't is one way to disambiguate.