"I am standing opposite you."
Translation:Stoję naprzeciwko was.
There are two possible options for Genitive of singular 'you': accented "ciebie" and neutral 'cię'. Only the accented form should be at the end of the sentence (unless the sentence is literally 2, sometimes 3 words long). So it would be "Stoję naprzeciwko ciebie".
It's hard to say when 'cię' would be okay for 3-word sentence and when it's not. It sounds wrong to me here.
I believe you mean "after prepositions". Let's take a look at the table for 3rd person pronouns here: https://en.wiktionary.org/wiki/Appendix:Polish_pronouns#Third-person_pronouns
Some of them have even 4 forms (this "-ń"), but let's keep it aside, it's rare and kinda dated/poetic.
So let's look at Genitive masculine. A simple sentence like "I am looking for him" will be in Polish "Szukam go". "go" is this basic, neutral form of the pronoun.
Now, let's contrast "him" and someone else, like simply "you". "I am looking for him, not for you!" will be "Szukam jego, a nie ciebie!" with both pronouns taking the emphasized form. If no separate emphasized form exists for a given example, then you just use the basic one, of course.
And finally we have the "n-forms" to use after a preposition, provided of course that such a separate form exist - you can see that they exist only for Genitive/Dative/Accusative in 3rd person. This would be like "I am standing opposite him" = "Stoję naprzeciwko niego".
If it's written in a direct message to 'you', then yes, it should be with a capital P. Like, imagine sending a text to someone you were supposed to meet (formal settings).
But if you find it in a dialogue in a book or movie subtitles, then it is just 'writing down what someone said' instead of addressing someone directly, and therefore it's not capitalized. That's why no Duolingo sentence (I think...) capitalizes any form of you/your, because those are sentences without context.