Learn Hardcore Croatian: Vidim ju u gradu poslije posla.

ju / je are the unstressed clitic forms of “her” (accusative singular of ona).
nju is the stressed (full) form, used for emphasis or after prepositions.

All of these can be correct depending on nuance and position:

Neutral: Vidim je / Vidim ju = I see her.
Emphatic: Vidim nju (implies “her (and not someone else)”).
After a preposition you must use the full form: za nju, bez nje, s njom (not clitics).

In Croatian, both je and ju are common; ju is very natural right after the verb.

Where can the clitic je/ju go in the sentence?

Croatian clitics prefer “second position” in the clause (after the first stressed word or phrase).

Neutral: Vidim je u gradu poslije posla.
If you front a place phrase: U gradu je vidim poslije posla.
If you front a time phrase: Poslije posla je vidim u gradu.

Note:

Clitics like je/ju almost never start a sentence.
In second position after a fronted phrase, je is the safer choice; ju is heard but is less standard there.
The stressed form nju can go anywhere for emphasis: Nju vidim u gradu…

Why is it u gradu and not u grad?

Because of the meaning:

u + locative = location (where): u gradu = in the city.
u + accusative = motion (into): u grad = into/to the city.

Your sentence describes location, so u gradu (locative) is required.

Which cases are gradu and posla, and why?

gradu is locative singular of grad, required by u when it means “in.”
posla is genitive singular of posao, required by poslije (“after”), which always takes the genitive.

So: u gradu (in the city) + poslije posla (after work).

Is poslije the only way to say “after”? What about nakon or posle?

poslije and nakon both mean “after” in Croatian; nakon can sound a bit more formal. Both take the genitive: poslije posla / nakon posla.
posle is typical of Serbian or regional speech; it’s not standard Croatian.
Don’t confuse with iza, which primarily means “behind” (spatial), though it can mean “after” in time in some expressions.

Is the order “place + time” obligatory? Could I say Vidim je poslije posla u gradu?

Both orders are fine. Croatian is flexible with adverbials:

Vidim je u gradu poslije posla (place then time) — very natural.
Vidim je poslije posla u gradu (time then place) — also natural.

Word order can be used for emphasis: put first what you want to highlight.

Does Vidim mean “I see” or “I’m seeing” (arranged meeting)?

Croatian has one present tense. Vidim can mean:

Immediate perception/state: “I see (her).”
In colloquial scheduling: “I’m seeing her (later),” but this is less common.

For a planned future meeting, it’s clearer to use the future or a verb like “to meet”:

Vidjet ću je u gradu poslije posla. (I will see her…)
Naći ćemo se u gradu poslije posla. (We’ll meet…)

What’s the difference between vidjeti and viđati?

vidjeti (perfective): a single act or immediate perception. Present often describes what you perceive now: Vidim je.
viđati (imperfective): habitual/repeated action. Često je viđam u gradu. (I often see her in the city.)

Can I drop the object pronoun and just say Vidim u gradu?

No. If the object is understood from context, you can omit it only if you also omit the verb’s object entirely (e.g., answering a question with just Vidim). In a full sentence with a transitive verb like vidjeti, you normally include the object: Vidim je/ju or use a noun: Vidim Anu u gradu.

Why is it poslije posla and not poslije posao?

Because poslije requires the genitive case, and posao has an irregular genitive singular: posla. So: poslije posla = after work.

Why u gradu but na selu and na poslu?

Croatian uses u and na in fixed collocations:

u gradu (in the city), u školi (in school), u kući (in the house).
na selu (in the countryside), na poslu (at work), na fakultetu (at university), na trgu (in the square).

You simply learn which nouns prefer u or na in these idiomatic combinations.

If I start the sentence with a time phrase, where does the clitic go?

Put the clitic in second position:

Poslije posla je vidim u gradu. Do not start with the clitic: ✗ Je vidim…

How do I emphasize “her” or “after work”?

Emphasize “her”: use the stressed form or front it: Nju vidim u gradu poslije posla.
Emphasize “after work”: front the time phrase: Poslije posla je vidim u gradu.

Any common mistakes to watch out for?

Spelling: it’s poslije, not “poslje.”
Don’t use u grad for location; use u gradu. Use u grad only for motion.
Don’t start a sentence with clitics like je/ju; place them in second position.
After prepositions, don’t use clitics: say za nju, not ✗ za je/ju.

Pronunciation tips for poslije and ju?

lj in poslije is a single sound (like the “lli” in Italian “famiglia”). Say it as one smooth consonant: “pos-lye.”
ju sounds like “yoo.” So Vidim ju ≈ “VEE-deem yoo.”