Nie rozmawiaj z obcym.

Because rozmawiaj is the imperative form: it means talk! / speak!

• rozmawiać = to talk / to converse (the dictionary form)
• rozmawiaj = talk! (command to one person, informal)

With nie, it becomes a negative command:

• Nie rozmawiaj = Don’t talk

So the sentence is giving an instruction, not just naming the action.

Polish usually does not need to say the subject pronoun if it is clear from the verb form.

In rozmawiaj, the ending already tells you this is a command to one person in the informal singular sense, so an English you is understood automatically.

So:

• Nie rozmawiaj = Don’t talk / Don’t talk you is not how Polish works
• The you is built into the verb form

Because in Polish, nie is normally written as a separate word before verbs.

So:

• nie rozmawiaj = correct
• nierozmawiaj = incorrect

This is a very common pattern:

• Nie rób tego. = Don’t do that.
• Nie idź tam. = Don’t go there.

Because the preposition z meaning with requires the instrumental case.

The adjective obcy changes form in the instrumental singular:

• obcy = basic form
• obcym = instrumental form

Since rozmawiać z kimś means to talk with someone, Polish uses:

• z + instrumental

So:

• z obcym = with a stranger / with a foreign person / with someone unknown

Obcym is the adjective obcy used like a noun.

The adjective obcy can mean things like:

• strange
• foreign
• unknown
• unfamiliar

In this sentence, it means something like a stranger or an unknown person.

Polish often does this: an adjective can stand on its own when the noun is understood.

So z obcym is short for something like:

• z obcym człowiekiem = with a stranger / with an unknown person
• possibly z obcym mężczyzną depending on context, though here it is usually more general

Yes, that is a very good question. Obcy has a wider meaning than the English word stranger.

Depending on context, obcy can mean:

• foreign
• strange
• not one of us
• unknown
• a stranger

In Nie rozmawiaj z obcym, the natural meaning is Don’t talk to strangers or Don’t talk to a stranger.

So the exact English translation depends on context, but in a warning like this, obcy is understood as an unknown person.

Because Polish and English do not always use the same prepositions.

In Polish, the standard pattern is:

• rozmawiać z kimś = literally to talk with someone

English often says:

• talk to someone
• talk with someone

But Polish uses z here, not a direct equivalent of English to.

So even if the English meaning shown is Don’t talk to a stranger, the Polish structure is based on with:

• rozmawiać z kimś

Grammatically, it is singular:

• z obcym = with a stranger / with an unknown person

So word-for-word it is singular.

However, in real usage, especially in warnings, Polish singular can sometimes express a general idea, much like English can say:

• Don’t talk to a stranger
• Don’t talk to strangers

Both can fit the general meaning, but the Polish form here is singular.

It is informal singular.

That means it is said to one person using the ty form, for example:

• to a child
• to a friend
• to someone you address informally

It is not the formal version.

Possible alternatives:

• Nie rozmawiajcie z obcymi. = Don’t talk to strangers (informal plural)
• Proszę nie rozmawiać z obcymi. = Please do not talk to strangers (more formal / polite)
• Niech pan/pani nie rozmawia z obcymi. = formal singular

The imperative of rozmawiać is formed as rozmawiaj.

This is just the standard imperative pattern for many verbs ending in -ać:

• czytać → czytaj
• pytac would actually be pytać → pytaj
• rozmawiać → rozmawiaj

So rozmawiaj is the correct command form.

The -aj ending is very common in imperatives of this type.

Not if you want the same natural meaning.

• rozmawiać = to have a conversation / to talk
• mówić = to speak / to say

In Polish, the normal verb for talk with someone is rozmawiać z kimś.

Mówić z kimś exists in some contexts or regional usage, but it is not the best standard choice here.
So:

• Nie rozmawiaj z obcym. = natural
• Nie mów z obcym. = not the usual standard way to say this

Yes, and many learners notice this.

• nieznajomy = stranger / unknown person
• obcy = strange / foreign / unknown / outsider

In many contexts, nieznajomy is a very direct equivalent of stranger.
So:

• Nie rozmawiaj z nieznajomym. = also a natural sentence

The nuance is that obcy can feel a bit broader, while nieznajomy focuses more specifically on someone you do not know.

In warnings to children, both ideas are understandable, though the exact preferred wording may depend on context.

Formally, obcym is a masculine singular instrumental form, but in a sentence like this it is often understood more generically.

Polish often uses masculine forms in a general sense when the exact person is not specified.

If you specifically wanted to mention a woman, you would normally make that explicit:

• Nie rozmawiaj z obcą kobietą. = Don’t talk to a strange / unknown woman

But in Nie rozmawiaj z obcym, the idea is usually simply with a stranger, not necessarily with a male stranger only.

Rozmawiać is imperfective, and that is normal here.

Negative commands often use the imperfective when you mean:

• don’t do this in general
• don’t start doing this
• don’t be doing this

So Nie rozmawiaj z obcym is a general prohibition:
Don’t talk to a stranger / Don’t talk to strangers.

A perfective partner is less central here because the point is not one completed conversation, but the general action of engaging in conversation.

Yes, it can sound natural on its own, especially as a warning or instruction.

For example, a parent might say it to a child, or it might appear in safety advice.

Still, in real modern Polish, you might often also hear a plural generic version:

• Nie rozmawiaj z obcymi. = Don’t talk to strangers

That version can sound even more like a general rule.
But the singular sentence you were given is still grammatically correct and understandable.